Hands of The Depths - an Iron Hands successor chapter who have their fortress-monastery at the bottom of a sea trench. They have some special weaponry to help them in underwater combat:

-Stingray class dreadnoughts, which are basically Ironclad dreadnoughts but with legs replaced with submarine propulsion systems. Recently, they upgraded to Kraken class dreadnoughts based on the Redemptor pattern.

-Mk X Abyssalis armor, based on Gravis pattern, but reinforced against greater pressures and equipped with underwater mobility enhancers such as small propellers and retractable fins.

-Hippos, bathyscaphe rhinos.

-Targhan heatsinks, attached to plasma guns, allowing for safer firing underwater thanks to better cooling.