Problem is Scouts ARE in armour.
Where a Space Marines armour is hermetically sealed and will be just like the old diving "dry suits" the scout armour will assist the swimmer in the same way a large brick tied around their neck would.
Well I'd imagine they would have armor variant to use like an armored wetsuit or just go in their undies. The other option is they just say, "Screw it!" and swim with the gear on.
I've seen videos of US and UK Special Forces doing combat swims with ammo and other gear on them that would drop your average person under water, so a SM Scout could probably swim fairly well with a brick around his neck.
But how big of a brick are we talkin'?