Look for some feedback or suggestions on how to implement the following scenario.

A party of five is split amongst two log rafts in the middle of a river with 55 yards from either side is the shore on a DC 15 strength rapids that is getting stronger by the minute.  They are navigating via oars two per raft but one of the rafts has lost an oar due to circumstances from a previous encounter on the river.   The river from this point on is littered with debris and rocks of various sizes as it carries the rafts to a waterfall (25-foot drop) 100-150 yards away.   

And that is where I cut the game for the next session.  I did that for two reasons first it was a perfect 'cliffhanger' moment to end on and second I wanted to work out some type of challenge that the players need to work through to either get to shore while avoiding the rocks and debris before the raft goes over the falls.  I have not found any good navigation and movement rules for small watercraft so I've been doing kludgy homebrew with strength saving throws to avoid obstacles but now I'm trying to wrap up the tension and involve the players with their dice rolls.

And I'm blanking.

My ultimate goal is a way to have the roles mean something as they try to move along the fast current trying to get to shore on a successful roll where failures could push them more to the center of the river and closer to the falls all the while dodging debris.  I don't want it to be a monotonous dice chucking but trying to simulate moving down rapids on a large river so I picture over four to six rounds is the ultimate end where the falls are located at and each round doing some form of navigation check against the DC of the river to push the raft off middle and closer to shore at certain increments. The possibility that movement distance shore is based on each person who can row. I have only five people on two rafts but only three oars.  For the debris, I'm thinking of lifting the Complications from the Chase Rules in the DMG to tell when if something could come up the party rafts needs to avoid.

Checks I'm looking at that I could use some feedback or suggestions on

Note: Using Rowboat Stat block for the rafts.

Move toward shore 
* Not sure where to go with this maybe strength but what else could be used for navigation Dex or Wis? DC against rapid strength or some other measurement to define a contest. 
--Success -- Move X Distance
--Failure -- No change move X distance further downriver.

Avoid Debris - Strength SAVING THROW Check and/or  Group SAVING THROW Check(Strength) per-oar
-- Success -- Avoided debris
-- Failure --  Party makes Dex Saving Throw or thrown from a raft into the water.   Raft makes Con contest roll against the size of debris on failure the raft takes damage 

Does anyone have the Vehicles (Water) skill?  it comes with Sailor background, a popular choice because you get perception.  I would not use the Navigation skill, that is more like using a sextant to know where you are.

I would definitely have them make a Vehicle (Water) check, one per oar.   In this situation I would let them make their choice of Strength or Dex based skill.  Not Wisdom, they are reacting to quickly to think it through.

I would rule that it increases. 

Round

1. Move 5 ft toward water fall
2. Move 10 ft toward water fall
3. Move 15 ft toward water fall
4. Move 20 ft toward water fall
5. Move 25 ft toward water fall
6. Move 30 ft toward water fall  and go over the water fall (unless you have done something else.

Moving towards Shore, I would make it based on how MUCH they succeed.  The river is 110 yards wide,  so they have to move 55*3 = 165 ft.

Roll d20 Vehicle (Water), check.  DC - 10 (easy) at first.  Move it to DC 15 on round 3, and 20 on round 5. Distance moved = score x 5 ft.   On a roll of 14, you move 4*5= 20 ft toward shore.  Roll of 19, you move 9*5=45  to shore.   Roll of exactly 10, you stay where you are, roll of 9, you move 1*5 =  5 ft back towards the center of the river. 

Oh, and a natural 1 or a score of means they have to make a Strength check and on a failure of the Strength check,  either a) if they have 2 oars, lose one of them or b) if they have one oar, they go flying from the raft and are in the water.

I would not have them roll saves for debris - it's the boat that hits it, not them.   That is what the natural 1 is for.

If they are about to go over (round 6), and to far to make it to shore,  I would give them one last chance to GROUND the rock on a large rock within 20 of the falls.   To do that require two rolls, one a DC 20 to land it, and again a DC 15 to stick the landing.   Failure here means they are going over the falls.

Quote from Mog_Dracov >>

Does anyone have the Vehicles (Water) skill?  it comes with Sailor background, a popular choice because you get perception.  I would not use the Navigation skill, that is more like using a sextant to know where you are.

I would definitely have them make a Vehicle (Water) check, one per oar.   In this situation I would let them make their choice of Strength or Dex based skill.  Not Wisdom, they are reacting to quickly to think it through.

I would rule that it increases. 

Round

1. Move 5 ft toward water fall
2. Move 10 ft toward water fall
3. Move 15 ft toward water fall
4. Move 20 ft toward water fall
5. Move 25 ft toward water fall
6. Move 30 ft toward water fall  and go over the water fall (unless you have done something else.

Moving towards Shore, I would make it based on how MUCH they succeed.  The river is 110 yards wide,  so they have to move 55*3 = 165 ft.

Roll d20 Vehicle (Water), check.  DC - 10 (easy) at first.  Move it to DC 15 on round 3, and 20 on round 5. Distance moved = score x 5 ft.   On a roll of 14, you move 4*5= 20 ft toward shore.  Roll of 19, you move 9*5=45  to shore.   Roll of exactly 10, you stay where you are, roll of 9, you move 1*5 =  5 ft back towards the center of the river. 

Oh, and a natural 1 or a score of means they have to make a Strength check and on a failure of the Strength check,  either a) if they have 2 oars, lose one of them or b) if they have one oar, they go flying from the raft and are in the water.

I would not have them roll saves for debris - it's the boat that hits it, not them.   That is what the natural 1 is for.

If they are about to go over (round 6), and to far to make it to shore,  I would give them one last chance to GROUND the rock on a large rock within 20 of the falls.   To do that require two rolls, one a DC 20 to land it, and again a DC 15 to stick the landing.   Failure here means they are going over the falls.

Dang!  I really like this suggestion.  Thanks!  I'm going to prototype it out to see if I can run into any issues along the way.  

To some of your points.

1) Vehicles (Water), no one has that skill and I have not been able to find much on it outside of "Vehicle Proficiency. If you have proficiency with a certain kind of vehicle (land or water), you can add your proficiency bonus to any check you make to control that kind of vehicle in difficult circumstances." in the PHB.   I never understood what that skill/proficiency gives to the user outside of just adding the bonus value.

Since no one has it I guess it would be a matter of either STR or DEX.  I struggle to determine which ability to use while I feel STR is more the physical labor of using the oar to put power into moving the raft I guess DEX is more of a reaction but how would I view it as the movement of the craft itself.

2) "I would not have them roll saves for debris - it's the boat that hits it, not them."  Yeah, I can understand that I don't think I was doing a good just separating raft from people and just mushing them up.  So maybe I address it this way.

Avoid Debris (RAFT) - Dexterity Check. (I would roll or give honors to the player in each raft to roll for it.)
-- Success -- Avoided debris
-- Failure --  Raft makes Con contest roll against the size of debris on failure the raft takes damage.  (Or maybe just take damage on the fail and no bother with the raft con roll.  I was wondering if larger the debris the more of an impact since it is a flat raft it would have by chancing knocking out players off the raft, maybe Acrobatics DC levels from Medium on.) 

Size/Bludgeoning Damage
Tiny/Small 1d6
Medium 1d10
Large 4d10
Huge 8d10
Gargantuan 16d10

This part I cobbled together from a UA doc from Wizards: https://media.wizards.com/2018/dnd/downloads/UA_ShipsSea.pdf 

To answer question 1. You said it, proficiency lets you ad your proficiency bonus. In the game with bounded accuracy, it makes a big difference. There's this idea that there's no such thing as a skill check, there's only a specific kind of ability check where you can add you proficiency. If it helps to think of it that way. As far as what ability people would use in this case, I think your idea of str or dex is a good idea. Though I could also see someone making an argument for wisdom saying they are able to look ahead in the stream and perceive and anticipate what the current will do and then adjust accordingly.

For 2. You could do a group skill check (everyone rolls, and if you have more success than fail, they pass the check. And I think just having the raft take damage s the way to go, but I'd also use the item hardness rules, which I can't find right now, sorry, but it would basically give the raft some DR based on what it's made of.

Quote from Xalthu >>

To answer question 1. You said it, proficiency lets you ad your proficiency bonus. In the game with bounded accuracy, it makes a big difference. There's this idea that there's no such thing as a skill check, there's only a specific kind of ability check where you can add you proficiency. If it helps to think of it that way. As far as what ability people would use in this case, I think your idea of str or dex is a good idea. Though I could also see someone making an argument for wisdom saying they are able to look ahead in the stream and perceive and anticipate what the current will do and then adjust accordingly.

For 2. You could do a group skill check (everyone rolls, and if you have more success than fail, they pass the check. And I think just having the raft take damage s the way to go, but I'd also use the item hardness rules, which I can't find right now, sorry, but it would basically give the raft some DR based on what it's made of.

 I found the Object HP/AC in the DMG https://www.dndbeyond.com/sources/dmg/running-the-game#Objects which I think what you were referring to which is something I should check out to see if anything can be applied.

Math is fun.  So I threw the rules into Excel and tried to model the encounter.

Party: four 2nd level characters with an average of 13 STR, 15 DEX, and 13 WIS,
Note: NPC fifth member but has a 7(str), 12(dex), 16(wis).

Using the initial data the model returned at the rafts was going over the falls in pretty much in every situation.  Those DC 20 were killer on the rolls, though to be closer to the falls the harder it would be so I feel it was a realistic progression.   Still, it was six rounds of rolling to the same outcome and hardly any progression to the shore. 

Using that data I made some adjustments to with river as well as DC.

Thinned out the river to shore

Since they are at least 100 yards off to the falls I adjusted the DC better to reflect the character's possible dice rolls and still have a challenge.  Again, while realistic the 20 had to go as the model showed it was what threw the members over the falls no matter how good they were beforehand.

Outcomes: I looked at all possible situations with the oars even though Raft 1 has only 1 and raft two has both of theirs, just to gauge if the players tried to improvise what could be a possible result.  

Raft 1:  All Three Members with oars 

Raft 1: If two oars (Original Equipment loadout) 

Raft 1: Single Oar (Current equipment on a raft. As they already lost one.)

Raft 1: Single Oar (Current equipment on a raft. As they already lost one.)

Raft 2:  All Two Members with oars 

Raft 1: Single Oar 

Raft 1: Single Oar 

Data, The rolls per attribute to DC with Travel distance.