No livro Horus Heresy " Fulgrim ", vemos brigas subaquáticas envolvendo os Fuzileiros Espaciais (Astartes / Crianças do Imperador ) no mundo aquático de Laeran.
Existem vários compromissos submarinos.
The war for Laeran was fought across many varied battlefields, the warriors of the Emperor’s Children fighting on floating atolls and through the ruins of ancient structures that reared from the oceans, while foaming breakers crashed against walls that had once stood thousands of metres in the air.
Underwater cites were discovered within days of the campaign’s opening and detachments of Astartes took the fight to the abyssal darkness of undersea trenches, smashing into structures that had never known the touch of sunlight, in specially modified boarding torpedoes fired from cruisers hovering above the sea.
De acordo com o Codex: Space Wolves , os Space Marines são capazes de respirar água
Regardless of his Chapter, each Space Marine has a secondary heart and an auxiliary multi-lung, allowing him to survive massive damage to the torso and fight on regardless. He can sustain and swiftly recover from wounds that would cripple or kill a mortal man, hibernate for decades, breathe water or poison with no ill effects, spit acid at his foes and even absorb an enemy’s memories and thoughts by eating and assimilating their flesh. Space Marines are the creations of the Emperor himself, and they share a measure of his power.
E dá outro exemplo de uma batalha no fundo do mar, desta vez envolvendo a Legião dos Lobos Espaciais . Evidentemente, a Guarda Imperial não estava à altura da tarefa, então eles precisavam de uma legião da Space Marine para fazer o trabalho.
The Tau Empire, encroaching ever further into Imperial space, mines the bed of the ocean world of Kvariam Alpha. It takes a full decade for the domes and corridors that stretch along the ocean floor to be discovered by an augur probe. With the Imperial Guard unable to engage the Tau forces so far below the sea, the Space Wolves are called in.
...
As the tanks near the alien domes and spires of the Tau cities to disgorge their Terminator-armoured passengers, circular portals in each section fan open and teams of graceful, propeller-tailed battlesuits move to intercept the slowly approaching armoured column. A desperate, silent undersea battle erupts, but despite the bewildering array of sleek torpedo-armed craft brought to war against them the Space Wolves still manage to shatter the domes of the Tau mining operations. The sea around them is choked with corpses before the Tau finally flee.