The problem is that a naval manpower pool doesn't really represent what is really going on. Naval manpower is the exact same thing as normal manpower: active, fit, service-age males who could join the force and fight.
Most of the things that made sailors specialist can't be represented by a pool, but would need a system of profession or education ala Vicky POPs or HoI education laws to actually represent it. In truth, the limiting factor to navies was never manpower: it was time and money. For example, during the Napoleonic Wars Britain did't impress American sailors because it lacked men who could be sailors or lacked the ability to train sailors, it impressed sailors because adequately training sailors took lots of time and money, and at the moment sailors were in drastic need and time was far from plentiful.
Thus, I think EU actually adequately represents ships well in this regard. They are relatively expensive and take quite a bit of time to build. Which represents not just the time and money it takes to build a ship, but also the time and money it takes to assemble and train the crew. I also think heavy ships should take longer to build than they do, but the general idea of it is right as it is, I think.
There actually literally were such pools, a naval manpower pool would be comprised of
registered sailors, not a general pool that you turn into soldier as the manpower pool is. The accumulation of said sailors would be influenced by conscription programs, the French instituted the
conscription maritime in 1681, the Spanish had
la leva. At the start of the Napoleonic wars, the French had 60,000 registered sailors, with divisions within of trained specialists (naval gunners, bombardiers,) who in peace time guarded the docks, and at war formed the nucleus crew of ships; the general pool of able-bodied seamen would be drawn up and divied out for war. The British did not
on paper have as efficient of a system as the French; ad-hoc impressement of merchant sailors, in the 1790s they also instituted the quota act, which substantially aided in the sailor pool. But British naval recruitment is a big mess. The Royal Navy was enormous, in 1796 there was 114 thousand men in the navy.
The Spanish were deeply constrained by their lack of sailors; in 1787 they had 53,147 registered mariners, and needed 89 thousand to man their fleets fully. They could build all the ships they wanted with their colonial wealth (and did, and the British prized Spanish ship-of-the-line captures), but the acute shortage of sailors had a disastrous effect on their navy.
I don't think the existing scheme is very good, you can lose your entire fleet and it'll make your fighting efficiency
better, the next time. The limits are ducats, time, and provinces. But you you can just borrow a sack of ducats, and build a ship of the line in every coastal province you own, (Victoria does this better with big naval bases) and in a few years you've rebuilt your entire fleet, better than before; just gather it up.
Adding time to construction is a good solution as it stands now though, it took years to construct some ships of the line, though galleys shouldn't take more than a year to build.
...well, composite bowmen were hardly a naval thing.... They were a land unit that had uses at sea. If they had lost a massive number of composite bowmen on land, they would have faced the exact same issue. Archers in general take a lot of time and effort to train, regardless of the kind of bow they use (although, granted, some bows are easier to learn than others). This is why crossbows and then guns became so prevalent in the first place: because they were cheap and easy to make and train.
Composite archers were the primary offensive means in the Ottoman navy at the time, losing all of them gutted their primary means of actually fighting at sea. As I said they rebuilt most of their fleet but they avoided confrontation for years.
