So the basics: My group are about to go on a sea voyage to get to their next location. After speaking with them they don't really want to just skim over it, nor get pulled in to heavily such as a busy pirate port, but would like it to be left to chance when it comes to what could happen. I have in mind a table to roll, with simple weather events that could speed up or slow down the journey, to combat and non combat encounters.

I have found some lists of encounters using google and had some ideas of my own. I was just wandering if any DMs out there had suggestions of events that they have run for their groups.

Suggestions for events that have no impact on the journey are welcome as well. Things that are just a unique moment for the individuals on watch at that time, such as a meteor shower as an example.

I have also seen various rules for ship health/combat and would like to know what rules you have used that worked well or had flaws to avoid/adjust.

How about something along the lines of "the boat breaks", some rigging snaps or maybe the anchor fails. Immobilizing the PCs until they fix it

I'd highly recommend purchasing "Ghosts of Saltmarsh", the Appendix A chock fulla info and options for ships, better defined roles for crew (whether they are NPCs or the Party wants to fill a role), how to make crew morale matter and how to influence it, plenty of encounter random generators not only involving sea monsters but hazards like storrmy weather, outbreaks of fire, rat or pests infestations, ideas for scenarios from the more exploratory exotic to the interestingly dangerous, etc etc just a buncha fun stuff.

I ran a few sessions pre pandemic (whenever Ghosts was released) on the high seas and it was a blast.

I'd heavily recommend that if the Party is just tagging along on a ship as passengers, and if you suspect to last more than several sessions, to really develop personalities and such for the crew. A ship's cook can really spice up an otherwise uneventful evening by telling exaggerated (or are they?) stories about legendary monsters, mysterious islands, alleged sunken treasures... The crew might also like to gamble, fist fights break out, petty pranks, personal items go "missing", all impact overall crew morale, but luckily a Bard willing to lay on the charm everybody up over a hit bowl of shrimp stew and a set of bongo drums might just prevent the boatswain from chuckin' the quartermaster overboard.

Otherwise, some feedback I shouldn't forget to mention is to try to respect the Party's roles vs. The Crew's roles in whatever type of potential combat that might come up. Meaning, if one of them isn't the Captain, what influence could they  (or should they?) have in the boat turning port vs. starboard, full speed or drop anchor when needed? I also heavily streamlined all of the ships' weapons and crew roles b/c I found out very quick it wasn't any fun for anyone to make 1000 rolls for dozens of crew and separate cannons and damage or else it could take all night. Wargaming mechanics (of your choice) could work if you thought that's interesring, but I found it took up too much my time and bookkeeping getting in the way of the players actually taking their own turns.

There’s plenty of water based monsters, depending on the party’s level.

It really depends on how much you want to make of it. Is it a single session with a fight or two, or do you want to make a small arc of it. Either way, personally, I’m not a fan of random encounters. Find a way to tie it to the larger plot. Maybe some of these ocean dwellers have an item that the party will need, even if they don’t know it yet. Or one or two of the characters have a personal story that will tie in: a brother or cousin abducted by pirates kind of thing. 

Sahuagin attack! What's wrong with that? I mean, you can have some sahuagin attack your ship and try to have like a tower defense on the decks! Sounds good, right?

One that I had a lot of fun with was a storm. Turn it into a skill challenge where the actions of the PCs make the difference between the ship wrecking onto a deserted island and the ship continuing to float on its own. Have them describe what they do and make skill checks to determine if they manage to help the crew or not. 

Run the traditional “damsel in distress” scenario. But the damsel is part of an ambush and is either an illusion or one of the ambushers. 

Have something climb onto the ship. It can be anything from a sea lion escaping from an orca to an aquatic troll who smells dinner. 

They see a sail in the distance. And when they get closer they see that the ship is flying a flag with a skull and crossbones on it. Or the ship is badly damaged and will obviously sink before it gets to land without help. 

You could run the voyage as a skill challenge.