Lately I've been thinking about game mechanics that were added/refined in Anno 1800, and how they might be improved further in subsequent games. One of the more interesting thoughts I had while spitballing was an expansion of the workforce mechanic to also apply to ships.
What if, upon upgrading a farmer house, you could choose to upgrade them to either "workers" or "sailors"? Sailor workforce would live in something a bit more uniform and unglamorous than the worker houses.
So here's the meat of the concept: Sailors on an island would automatically be "used" upon construction of a new ship. Not an insane amount, maybe 25 sailors for a frigate and 50 for a SOTL, but only 5 for a schooner and 10 for a clipper.
I think it would be far too annoying to force the player to bring food along on the ships. . . but I think the sailor workforce should still provide a slight drain to tier 2 resources even while out at sea. How does your ship know what island to siphon these goods from? By establishing a Home Port, similar to how fleets in Stellaris have "Home bases" to which they return when you order the ship to repair or upgrade.
There could be a new pier building type specifically for loading/offloading sailors to an island where your ship would be "docked". This pier could be expanded with multiple docks to "dock" multiple ships.
Offloading crew at a ship's home port would increase consumption of tier 2 goods, thereby raising the morale of the crew (which slowly degrades over time at sea) and give your ship a temporary bonus to damage and/or speed. Think of it as your crew going on holiday tours and regaining strength by being "well-rested".
So, ideally, you'd be rotating ships out to holidays relatively frequently for your warships. This could be as easy as a setting called "patrol length", where you set minimum required morale for an individual ship or fleet. After your ship is nearing this minimum threshold, it would automatically return to its home port to refit and raise morale again.
The trade ship crews would go "on holiday" any time your ship is being offloaded at its pier, which, like the warship crews, will increase use of tier 2 goods for a short while on your island, but give your trade ship a small speed and unloading boost.
I think that there are several other "economic strategy games" such as Hearts of Iron that have a Manpower resource for vehicles, and it seems like the workforce element of islands would do pretty well translated into the naval aspect of the game too.
What does this change from an infrastructure perspective? A small amount of sailors, say from only a few trade ships, wouldn't make much of a dent in your island's good stores, but a massive fleet of 10+ SOTLs certainly would. So this could make large scale conflicts in the New World or Arctic particularly difficult to pull off in the early game, because if your warships in those regions dont have a place to make port, the morale will keep dropping until you send them back home to refit and give the crew a chance to get more morale from consumption of goods.
To me, this more accurately represents the challenges in operating a vast fleet in foreign waters. Supply chains during wartime are very important because of this. After all, a war can't be won with hungry sailors (or ships without ammunition for their weapons, but that's probably reaching unmanageable level of complexity in a game series like anno). Sure, you can set the ship/fleet's Home Port to a local island in the New World or Arctic, but keeping them supplied with goods, especially with large scale navies, would be difficult to pull off.