To give you a bit of help with this, here's a map that will give you a rough idea of the position of the ship in the cavern



As you can see, there are points where it should be easier to enter the ship without having to cross too much undead-infested water, though you'll still need to find a way to hoist yourselves up onto the rotting hulk without falling into the drink.

“I’m got at negotiations, even the aggressive or deceptive type but when it comes to tracking I can barely help.” Emrys said after the calls to lead on and his companions were fast to see his point. 

With their help it was relatively fast and surprisingly easy to dispatch the last two wraiths. In retrospect it shouldn’t have been. They had every advantage on their side, from numbers to range. The spirits showed signs of relief upon defeat and before long the way was free to the baiting strategy.

“Everyone that can attack from range stand ready.” He was fast to dive in the water and faster still while crossing it. Then as soon as the first undead was no further than a hundred and twenty feet he chanted. A brief word, if one could call it that. Its sound was not merely inhuman but unnatural, like no creature should be able to produce. Echoing it his eyes became as dark as the night without moon and his will directioned the Weave. Pure force moving towards a single target, equal parts attack and experiment. Would the enemy come after him? Would any other exanimate follow it? Whatever the case the halfling retreat and as far as his underwater speed would allow.

Notes: Emrys asks the party to ready a ranged attack against any undead that enters their range. He then swims until an undead is withing Eldritch Blast range and fires on a single target (21 to hit and 12 force damage, 13 to hit and 8 force damage). Soon after the attack Emrys moves back towards the tunnel and as much as his 40ft swimming speed allows. P.S. The map is gorgeous.

“Ready” Wade replies

Emrys' two bolts of force strike the undead solidly. It turns towards him and slowly begins trudging along the lakebed. You realize that heavy chains seem to be wrapped around its ankles, keeping it anchored to the bottom of the lake.

Fortunately, attacking a single undead does not seem to provoke all of the others to attack at once... but it would take a LONG time to clear them out one-by-one in this manner...

Areleo will be eager to fight but will stay back into he get the okay to charge.

Sayyida reiterates, " If they are that slow I can destroy them one on one to ease your passage...just say the word, Captain."

Sae draws out a fire arrow from her quiver and sights along it as Emrys dives into the lake, shaking her head as she sees the state of her quiver.  "Should have brought more arrows.," she grumbles.  "Would have if I'd known this underwater lair had so much bloody air in it."

As soon as she sees the undead that is pursuing Emrys break the surface in range of her bow, she will ignite the arrow and fire; or just shoot if it doesn't show above water at that point.

============

Held action: longbow attack  13, damage if hits 10 piercing + 6 sneak attack ... if Emrys is not within 5' she will use a BA [Insightful fighting 9, contested by undead's Deception ... sneak attack damage applies if she succeeds)

Yeah... there's no way that this undead is beating that nat 20 Sae got on Insightful Fighting.

The undead breaks the surface... and is immediately hit with Sae's arrow. Roll that bonus 1d4 fire damage, please.

Alright, I'll roll the bonus fire damage: 4

The undead staggers, but it's still trudging towards the party. It looks like one more solid hit could probably finish it off.

Et tu, Sayiida? Emrys thought as the paladin called him captain too. He considered her proposition as his eyes turned to the slow zombie, the situation proving that there was no need to worry about being swarmed by them. He saw her fight enough to trust she would be capable to keep the creatures at a distance but at the same time. But what if the undead suddenly changed their behavior?

 First we end this one, said the voice of his mind. Between the darkening of his eyes and the murmur of an arcane word he bent the Weave into force and shot his pursuer once again. The thing was so damaged by the barrage of attacks he and his friends managed to launch that the halfling allowed himself to be overconfident aand turned his attentions to the crew. The living one.

“There’s an easier way.” He said with his head out of the water. “They seem to react only if attacked or if we get too close. Clearly one at time. Chances are that if we navigate between them and out of melee range we can just reach the ship without fighting.” As soon as he said as much a darker thought crossed his head. “We can clean them one at time if we don’t think an enemy on the ship could control them.”

Notes: Emrys casts Eldritch Blast (21 to hit and 13 force damage, 15 to hit and 7 force damage) hopefully defeating the undead. He then leaves the question to the crew - moving between the undead mines or pick them up one at time? I'm trying to navigate between the zombies.

Undead Fortitude saves:

14

21

Quote from Mister_Whisker >>

Undead Fortitude saves:

14

21

 The undead gurgles disgustingly, then its body seemingly melts back into the putrid water as Emrys' bolts of eldritch energy free it from its unholy servitude.

Sayyida let a slight flicker of disappointment pass across her face but nodded to Emrys and began to set out for the ship.....

"Looks closer if we go around to the right, where the back part -- sorry Captain, Sayyida; where the ... stern? ... is nearest the cave wall.  The fewer zombies we have to steer around the better.  Looked like those chains were pretty long.  Your call, though."  Sae settles her bow across her shoulder where it will be out of her way but easy to reach if needed, takes a dagger in either hand, and follows Sayyida's lead towards the mouldering hulk.

@DM: How many undead can Emrys see with his devil's sight in the direction Sae suggested?

Quote from Makkiu >>

@DM: How many undead can Emrys see with his devil's sight in the direction Sae suggested?

Near the stern itself, there are around 4 undead visible. Now, you'll still need to get there as there are plenty of undead in the water near the ledge that you're going to need to follow in order to reach that point, and we remember what happened when Sae slipped on the wet rocks...

As for the total numbers in the lake... a lot.

Sayyida headed towards the ledge to follow it towards the stern hoping to lead others on a safe path....

Acrobatics? 4

( And rolls a 1)

Yeah...

Sayyida, being more focused on slaughtering the undead than navigating the slippery rocks, loses her footing and plunges into the water. Unsurprisingly, an undead is waiting to give her a big hug...

Undead's grapple attempt: 11

Seized by the undead and VERY annoyed at herself she hacked at the dead thing with her blade.

Attack: 18 Damage: 9

Attack: 20 Damage: 6