Board Thread:Warhammer 40k General Discussion/@comment-7499389-20130407223055/@comment-6078851-20140524012651

terminator and power armour are fully functional environment suits - they have a supply of air and can be sealed to function in space as well as underwater (although i've no idea how long the air supply might last).

whilst armoured it's concievable that astartes might be able to swim in power armour - astartes strength plus the armour's strength-boost plus a little bouyancy from the suit's internal air. but since the suit's are air and water-tight swimming in them is unecassary. there's no chance that they can swim in terminator armour though :)

as for unarmoured i see no logical reason why they wouldn't be able to swim, although in 40k insanely stranger things have happened and it's not inconcievable that in amongst the astartes' training regimens that someone forgot to add in their swimming certificates :P