Tips for naval battles

I’m a 3rd or 4th year DM running his second campaign, and in this campaign a prince sent them to guard cargo heading to a dwarven industrial kingdom. They lost their main iron-side cargo vessel due to a Daern’s instant fortress, but have a fleet protecting it to carry the cargo. Next session (I don’t know when the next session will be due to conflicting schedules), I plan on having a large pirate navy attack them for their loot (also gives me an excuse to use a pirate voice). The players are level 5 and this will be each of theirs first naval battle, this is my second but I was a player for the first one. Any tips on both balancing and running the combat would be greatly appreciated.

There are some ship combat rules in ghosts of saltmarsh.

However, I don't think they're all that important here. You have a large-scale fleet action going on, and, unless the PCs are quite high level, there's going to be very little they can do to influence things at scale.

Narrow things down to what they can influence. The ship battles in the background can just be scripted, possibly with points at which what the PCs are doing can influence them, either directly, or thematically (things go well/badly because the PCs are doing well/badly, not due to anything they did, but because they're the protagonists).

Meanwhile, they're presumably defending the cargo against boarders, probably several waves of them.

Agree with jl8e’s points above.  D&D is not really designed for large combat between armies.  Even WotC made a separate board game to serve as companion to their Dragonlance campaign book.

As noted above, your party’s goal should be something small and influential on the outcome.  Defending supplies is a fine option, but you could also do boarding the enemy ship to take out their leader.  

One thing not addressed above I want to flag - ship combat heavily favors range classes.  The distances are bigger with often limited obstacles, allowing casters to rain death while the threat approaches.  This is not fun for melee characters.  Whenever designing these encounters, you should come up with some way to make melee characters feel useful.

Quote from Caerwyn_Glyndwr >>

One thing not addressed above I want to flag - ship combat heavily favors range classes.  The distances are bigger with often limited obstacles, allowing casters to rain death while the threat approaches.  This is not fun for melee characters.  Whenever designing these encounters, you should come up with some way to make melee characters feel useful.

 The easiest way of implementing this is to have the ship be boarded by enemies. This also greatly simplifies your map situation since all you really need to deal with is the battle map for one ship (two ships if the boarding occurs because an enemy got close and sent over boarding parties).

We actually did something like this very recently. There were a total of 5 ships involved in the combat - my player's ship, 2 friendly ships, and 2 enemy ships. They had picked this fight and planned with their allies to attack these other ships. Their plan was to surround the enemy ships - so the map was just the player's ship plus 1 enemy ship, with the other ships just being handled narratively. If my players hadn't already wanted their melee fighters to board the enemy ship, I'd have had enemies do the boarding to make sure the melee fighters had something to do. I placed enough enemies on the ship to make it a difficult encounter, and then did lair action-style effects to represent the battle happening around them. I did contested D20 rolls with modifiers based on how much damage was done the previous round by each side to represent which side was "winning", so if the players had a really strong round, the battle overall was going in their favor, and vice-versa. There's so many people on a full-sized ship, it would be impractical to have that many NPCs/enemies for the DM to control, but this gave them the sense that there was still combat going on outside of what they were directly involved in.