I have a 1/2 gallon saltwater pool (about 2 litres for all you metric folks) and I see my crabs submerge all the way to the bottom multiple times a day. I've seen: Es, Straws, Violas, Indos, and Ruggies all do this. The only species I haven't seen do it is the PPs but that could be because I have no PPs in that tank. Some crabs will even sit on top of the airstone so that they can stay down longer. My straws will often stay down for a half hour when sitting on the airstone.
My experience tells me that they are not blinded by being submerged completely. Mine all seem able to find their way around just fine down there, even conducting 'conversations' with one another with their antennae when 2 or more are submerged at the same time. I think its a lot like my dog, he loves water and swimming but hates baths.
