Yes, there are many true freshwater crabs ... including fully aquatic species that don't need land access. They show up less regularly than red-claws (one or a few species of SE Asian Sesarmids) and fiddlers (
Uca spp.), both of which can tolerate freshwater but really need brackish and semi-aquatic conditions for long-term survival.
Here's one such fully aquatic freshwater species,
Potamonautes orbitospinus, the Malawi blue crab (from Lake Malawi, of course):
Cardisoma spp. like this
C. armatum (rainbow, Halloween, patriot, moon crabs) live in freshwater, but do best with a land area:
See
this page for the species you'll most likely encounter in stores.