Someone on Aquabid advertises "
Thai Micro Crabs" as being full freshwater and they apparently hide in the foliage. Don't need to breathe air and are really small.
Found
this also:
"Described in 1991,
Limnopilos naiyanetri Chaung & Ng, has only just been discovered by the hobby. Crabs are found in pure freshwater in the roots of floating vegetation, ie. water hyacinth (
Eichornia crassipes) and fine-leaved aquatic plants along the riparian zone (river banks). They have hairy bodies and appendages which collect mud and dirt and may function to trap food particles. They are not carnivorous, and behave like
Caridina shrimps in their scavenging behaviors, where care and feeding are probably the same. Suitable for nano-tanks, and is the first crab species available which has attributes suitable for such. In such tanks they will stay closely associated with mosses and fine leaved-plants. Max size 1 cm. (Photo C.Lukhaup, Text K.Diehl)."
You could also check out
Franks Aquarium. He has a lot of freshwater crabs. Not sure if any of them are non-terrestrial.
Hope this helps.