Because of their lack of armor I'd assume they would be quieter underwater and would be more agile compared to a full battle brother in war gear.
I've been slowly working on a homebrew chapter for a long time now and I figure why not shake up the scenery by including an underwater, camouflaged ass-kicking conducted by my Chapter's Scouts.
I thought about some (renegade?) IG Kasrkin doing some underwater surveying and swimming by SM scouts in seaweed camo, then getting their necks snapped in two. It would be dark and I would use prettier words to describe it, but I figured why not ask Heresy first?
Plus the scouts would probably have a breathing mask/goggles, flippers, combat knife, and maybe some kind of projectile weapon. (SPEAR GUNS AND SEAWEED MARINES!)