In the Forgotten Realms there are cannons.
Second, trigger happy mages.
If cannons are present, they typically will outrange low level mages. Without cannons, boarding actions would be more common, so not that much different than land actions. Just some extra complications.
Third, D&D being a group game where everyone wants in on the action instead of the captain making the decision (especially trigger happy mages).
Other RPGs make this work (like Star Finder) and it looks like the ship rules they released for comment give some nod to this. I think they missed a few roles in their nautical supplement, but the concept mostly works.
Forth, a completely different combat mechanic for naval engagements. You have flying, said mages and soon players will find out that with some magic they can attack ships from below which is very risk free.
It doesn't really require a different mechanic unless you want to make it a war game instead of an RPG. And they factors have been true with all version of D&D and in thousands of games run. So what?
And then there are all the different races with all their different takes and technology levels for their ships (at least when you follow FR lore).
So?
So my expectation is that any D&D nautical book will be a complete mess and requires an extra suspension of disbelieve to work. And that does not even address the problem of how to integrate ship vs ship combat where most of the time the entire party will have only one ship and thus one or two guys making the important decisions while the others are at best supporting roles into a game which spends a lot of effort on having everyone be effective in combat. That is a completely different mindset. Not to mention that naval combat usually requires several concepts D&D specifically ignores like facing.
You make this sound difficult -- it isn't. I have done it many, many times with every version of D&D. DMG does have rules for facing. It doesn't have very good rules for creatures and objects that are long compared to wide, but in ship-to-ship combat running it like a wargame would be kind of dull anyway.
And apart from combat a lot of problems of sailing like provisions, etc. are solved by low level spells and generally ignored for land travel anyway, so nothing you can fill a book with. This even extends to the motivation of being a pirate in the first place as money is plentiful in D&D and you get enough of it by simple adventuring. So why be a pirate which takes more effort and is not in any way less dangerous?
Yes, this is what magic is for, to solve problems. However, even in a magic rich world like the Forgotten Realms, not every ship will be staffed with mages and clerics.
Being a pirate is just a different from of adventuring. Also, I would assume the PCs are more likely fighting pirates and/or using nautical transportation to take them to an adventuring site.
Sindbad and the Seven Seas comes to mind.
I would be surprised if WotC manages to solve all this problems but I do not believe it.
Life is full of surprises. Hopefully this supplement will be one.
