One of the locations featured in the Dark beneath campaign for 5e and 5.5e is called the Hollow Sea. It’s an underground body of water comparable in size to a land locked sea on the surface. There are several sources of rules and mechanics for ship to ship combat and water vehicles for 5e and usable in 5.5e. Some of the preexisting rules are complex, others are simplified. What types of water vehicle mechanics do you prefer? If you prefer alternative rules or your opinion doesn't fit the available options, please feel free to leave an alternative response in the comments.