Just if title. I have some ideas like cannons deal 3d6 damage to the ship. The ship has an ac of 14 and has hp 125 and can only be damaged by siege weapons or explosives. Also there will be boarding party’s. Also rotatable canons the deal 2d6 damage. Does anyone have any ideas that can help me. I do want to include ships in combat.

Consider the ships more as a terrain feature than a direct tool. There should be a concept that the ships can move relative to each other, and relative to the ocean below. If it's a chase, falling in the sea means being left behind. If it's circling, or ramming, then the sea may be a viable place in which to do battle.

Cannons should do reasonable damage to ships and collateral damage to people. I'd consider having a cannon basically cast Shatter on wherever it's pointing (use the wording, not magic), so that it can deal more damage to inanimate objects and more of a splash to people nearby when it happens.

To balance this, I would make reloading a cannon take 2 actions, like a ballista. So action to fire, action to pack powder, action to prime. If you can shoot it every turn, then it's too powerful.

Ships will also have a powder store (the Magazine) which will, in a magical world, likely be heavily shielded with fireproofing, as any magical boarder who can shoot fire could detonate the ship otherwise. You may also choose to have other options for firing them.

Consider the different obvious ways of boarding and how hard they will be to execute, and set DCs for any not covered by the Jumping rules, ready for people to swing in the rigging or leap from the masts.

Give key features hitpoints and ACs and effects on the ship. This means that people can attack the Masts, Wheel, Rudder, hull, etc. rather than chipping off arbitrary hitpoints until the ship suddenly sinks. Want to slow a ship? take out the mast or sails. Want to stop it running away? Hit the rudder or the wheel. That sort of thing.

Main thing is to think "If I were a chaotic character, what would I do?", then write down the answer ahead of time!

There are stats for things like cannons in the DMG. Ships have stats in the PHB.

Now, obviously you can change the stats up to better fit your needs, but it gives a baseline to work from.

The main thing you need to do is make sure the players all have things to do. If it's just "one player steers the ship and the others load, aim, and fire the siege weapons", it's pretty dull. This likely means they're going to board or be boarded.

There are several videos on YouTube about ship combat mechanics. I'd watch a few and build your own based on taking what you like and ignoring what you don't.

Both DMGs have information on Ship stat blocks. It's pretty simple:

The ship on which the PCs are, has an initiative. The PCs will have initiative (if they aren't also acting as crew). This is the key thing so many DMs get wrong. DO NOT have the party be the only crew of the ship. You're just making too much work for yourself. That's not how these vehicles are designed. Also, don't include a boarding party bigger than the normal monster group size you'd run. So, it's fair enough to have a boarding party of six goblins or suchlike, but just don't try and have 80 pirates try to board...the ruleset isn't built for that.

This means that if you've got a Kraken attacking a ship, the ship (and ships crew) have a single initiative roll. The PCs also roll initiative. And of course, the Kraken rolls initiative. 

Swap out the ballistae (which typically deal 3d10 damage) for cannon and you've already got what you want. 

If you must have the PCs be controlling the ship I suggest that they have only a Keelboat, or Rowboat.  The PC controlling the ship chooses only from the list of actions on the ship's stat block. The rest of the PCs get to choose from their ordinary actions.

To be clear though, having the PCs be the 'crew' of the ship tends to suck real bad. So much easier to have them be able to hire a crew and ship than own one. 

Let's say you ignore the advice, fair enough, it's your table. But you'll be looking at 20 passengers (including the party) available to be in combat against the boarding party (likely of a similar size). In which case, I'd advise grouping the passengers into distinct groups (if you've got four players, take the remaining 16 passengers and have them break into four groups). Each passenger group acts as a single monster taking on a similarly sized enemy group. Give these groups maybe 80hp, with one of their members dying after each 20hp is lost. 

As I say though D&D 5e, and 5.5e SUCK at this kind of group combat. Some folks will willingly look past the flaws, but having played dozens of other systems and having tried over and over to get this kind of combat working it has two unavoidable consequences. The first, it requires a DM who is happy being overwhelmed by the sheer numbers. The second, it requires a party tolerant of so much dice rolling it'll slow down the combat to a painful grind. 

Running Pirate Borg taught me a lot about the better ways of running Ship to ship combat. I'd highly advise checking that out. It handles it so much better because it was designed for that.

I'm not saying don't do your idea, but given that the rules aren't built for it, make a compromise and ditch the boarding parties would be my best advice.

Quote from martintheactor >>

Let's say you ignore the advice, fair enough, it's your table. But you'll be looking at 20 passengers (including the party) available to be in combat against the boarding party (likely of a similar size). In which case, I'd advise grouping the passengers into distinct groups (if you've got four players, take the remaining 16 passengers and have them break into four groups). Each passenger group acts as a single monster taking on a similarly sized enemy group. Give these groups maybe 80hp, with one of their members dying after each 20hp is lost. ]

 I would say not even to bother with this. Combat entirely between NPCs doesn't need dice. If it's happening where the players can intervene, decide how you want it to go, and just narrate how it's going. Or base it on how the PCs' part of the fight is going -- if they're in trouble, everyone else is holding their own or winning, but if they're doing well, the NPCs are faring poorly, to put more pressure on the players.

Ghosts of Saltmarsh also has expanded ship types and rules for ships in combat.

If you want to go really in-depth, you can run ships stats (PHB and Ghosts of Saltmarsh I believe) and have them attack each other with various siege weapons (DMG). However, you can abstract it, and describe the conditions of the ships based on how well the battle is going. Think up a few ways for the players to destroy the other ship and be open to what they want to do. i would recommend having the stats handy, but unless the players directly attack the ship, don't worry about those and just run a combat. Having a random table you roll on once a round for events (cannon explosion, reinforcements, etc.) can add a special dynamic to the battle.

Thanks this has been very helpful