A guide to ship combat
From the introduction:
This module attempts to simulate ship combat as close as possible, while still maintaining the same speed and accessibility that made 5e such a success. The following rules are intended to be as easy to learn as possible and are easily graspable by any player that can understand basic combat.
Note that these rules are not intended to replace the traditional flow of combat that players would be accustomed to in order to focus their attention on two ships doing battle. Instead, these rules are intended to be layered on top of traditional combat to allow players on the micro scale to affect outcomes on the macro scale. As such, when reading this module, DMs are encouraged to imagine ways that the somewhat basic and straightforward combat between two ships can result in difficulties and challenges on the ships themselves that the players need to address. A fire breaking out on deck, men being tossed overboard or a band of Sahuagin climbing the sides of the ship; all of these are potential ways to turn the players from ordinary crewmen, into heroes of the seas.