While Astares armor may be air tight, I have to wonder how it deals with extremes of pressure? A current space suit would be crushed like a beer can at depth and I've never read anything about the various gaskets and connecting joints being pressure resistant. I would imagine the Astares physiology would be more resistant to oxygen toxicity at depth, or nitrogen narcosis, but anything more than 40 meters you still have an issue with cellular tissue struggling against rapidly expanding nitrogen in your body if you come to the surface.
Assuming the above weren't problems in whatever variant of armor being worn, you could probably use the same sort of manouveuring jets/thrusters underwater as you could in vaccum, or just have a propeller attachment to the, um, backpack
What sort of weapons would an Astares (or Scout) use underwater? At the least, if bolters worked, the concussive force of a near miss underwater would still cause significant damage. Does promethium still burn underwater? Plasma weapons would be pretty useless as the water absorbs a lot of energy and the steam displacement blows the Astares backwards.