I imagine it would depend on the planet the Scouts came from. Blood Angels and Salamanders wuld have trouble swimming at first. But once trained, a Marine(even a Scout) has sufficient strength to swim even with some gear. But since the fluff supports the game system, which has little to no rules about swimming, there will be little mention of it. But I also think Space Marines have access to small submersibles capable of infiltration. No proof of this but it makes sense. Can't exactly drop a Land Raider onto an ocean world, though a Stormraven might be able to be refitted for aquatic travel.
Black Tide has an underwater battle with full fledged Space Marines. Not the same but still a fun read.
"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 ..."