Oh, for goodness sake: they ate there in the book, and they work to function. Saying they "might as well not exist" is utter nonsense (goes for Saltmarsh or Spelljammer). It's D&D, not a naval simulation game. Quick and dirty gets the job done.
In Spelljammer the Ship Combat Rules are two pages. And what are they saying? Battle starts at either 250, 500 or 1000 feet. A spelljammer pilot can move the ship up its movement speed. When in 5 feet distance they can board another ship. They can crash into each other. They can use the mending spell. They may use side initiative.
That are literally all the combat rules outside of the ship statblocks.
What happens when we use these rules to actually do ship to ship combat?
First of all: Damage to ships really doesn't matter. It has no effect until a ship is reduced to 0 HP. So slowing down a ship by damaging it sails doesn't work. Killing its crewman to slow it down doesn't work. And the damage you are doing with ship board weapons would take for ever to reduce another ship to 0 HP. Like rounds in the 100th.
So destroying other ships is not really an option unless you get into fireball range.
So boarding should be the preferred tactic. Really, it is the only tactic that is supported by the rules. And only barley.
Scenario 1:
The ships are at 1000 feet distance. They want to fight each other. They both want to board each other. If both go full speed it will take, depending on the ships 10 round to reach each other. And in this 10 round, 8 to 9 round are out outside of any class ability to reach the other ship. So it is 8 to 8 boring rounds of "ship moves, fires weapons that don't do anything".
Now you can say you shouldn’t start a fight in a 1000 feet distance but that is what's written in the rules!
So, now let's say we have a galeon that wants to flee from a shrike- the biggest speed difference between two ships. Foe the shrike to catch up with a galeon it would take 26 rounds to get under 100 feet distance. 26 rounds of moving in a line each round, firing ship weapons and doing nothing else. If you have to ships closer in speed or even two ships equal in speed, you have an eternal chase.
And destroying a ship is like unrealstic. Within the far distances balistae only hit 1/3 of the times and only 2/3 of the times do enough damage to reach the damage threshold.
So if you use the Spelljammer Ship Combat Rules RAW you will always have super boring meaningless slog.
They work only in one scenario. That is: short distance between two ships (250 feet) that want to board each other.