It really depends on the engine you're using or the way you're making the game. In my experience, the biggest trove of assets with very high quality (4K) can be usually found in Fab for Unreal Engine. For example looking for ships:
Looking around I found an imitation of a Napoleonic ship and exports in Blender format to be used in other engines (it's also free :D):
If you don't want to be locked to mostly Unreal Engine, you can try cgtrader:
For example this one looks pretty cool:
I don't know how old you want the ships to be and how you're making the game but I'm sure you can find something there. If nothing satisfies you, and if you're willing to pay, you can always hire a 3D Modeler and give them the reference pictures to what you want them to create. However, that can get quite expensive, and if you're not sure of what you want you may even need an artist to draw concepts to bring your ideas to life. I'll leave the links to both Fab and cgtrader so that you can go look around. Good luck!
Fab: https://www.fab.com/search?listing_types=3d-model&q=ship
Cgtrader: https://www.cgtrader.com/3d-models/ship