Death Beneath the Waves

From publisher blurb:

There is no greater frontier of mystery and danger than the crushing black depths of the unforgiving sea. Underwater adventures send heroes into a hostile realm where ichthyoidal alien things prowl the cold currents, and the environs themselves choke off life. Running a game beneath the waves isn’t easy. Thankfully one of the world’s best fantasy game designers offers six pages of pdf packed with a plethora of tips, tricks, rules, and a brand new beastie to pump air into the gills of any GM brave enough to delve the watery deeps.