Depth Factor
Background
Movement
New and Modified Rules
New Equipment
Sample Ships and Creatures
Author's Notes

Depth Factor is a series of rule modifications to Jon Tuffey's Full Thrust rules that simulate submarine combat. It is NOT designed to be a particularly realistic Naval Combat game; Harpoon does a good enough job of that already. It is designed to be fast, fun, and easy to play (I hope).
A Word Document containing these rules is available here.