Board Thread:Warhammer 40k General Discussion/@comment-7499389-20130407223055/@comment-3580035-20130408023251

They can easily swim if they aren't wearing power armour, but sink if they are. However, Power Armour is waterproof (I think), so Space Marines can walk along a river bed (sea beds have too much pressure; as such only Terminators are able to walk there)