Hostile Waters for the new edition of RPG classic Twilight: 2000, is a massive modular expansion that includes everything you need to play in a marine environment. Hostile Waters is loosely based on Pirates of the Vistula for the first edition of Twilight: 2000 RPG, but designed to be used for any waterways.
This boxed set includes new combat and travel rules for marine environments, military watercraft large and small, new factions for the PCs to tangle with, new plots to unravel, and new scenario sites to explore – including the USS Maine Ohio-class nuclear submarine. It also describes the two sample waterways Vistula river and Göta Canal. Contents of the boxed set:

Full-color book stuffed with new rules and gaming material.
26 new encounter cards, for use on rivers or lakes.
14 modular battle maps for marine environments.
Five scenario site battle maps.
Battle map tokens for boats and ships of various sizes.
A huge double-sided full-color travel map for northern Poland and southern Sweden, connecting the maps in the core RPG set, format 864x558mm.