I would personally think that all Space Marines have some ability to swim. I believe in the Blood Angel Novels (in one of them) they attempt to attack a fortress run by Fabius of the Emperor's children, being boarded in their (whatever they called their boat) by a form of the Tyranid. I'm fairly sure there was swimming involved.
That's Black Tide
. I wouldn't say they were swimming. They magnetized their boots and walked on the hull. But I was surprised Power Armour is apparently buoyant.
"Fetch me another plaything. This one seems to have broken." -Urien Rakarth
Space Marine- "This planet is ours, witch."
Eldar- "No ... This planet is theirs ..."