Hi,
there are indeed a lot of crab species inhabiting the tidal areas, may it be rocks at the shoreline where species like Percnon, Grapsus, Pachygrapsus and others live which live partly in the wet and partly above the water. Then You have a large group of Mangrove crabs of the family Sesarmidae. Land crabs (family Gecarcinidae, this includes the Gecarcinus "Halloween crabs/Black back land crabs" and Cardisoma "Tricolor crabs") are getting very big and require terraria that should be 1,50 meters long at least – for one specimen! Species of the genus Gecarcinus can drown, Cardisoma breathes under water as good as on land, it has to be wet.
Of the mangrove crabs the "Red Thai crab" is the best known, Pseudosesarma moeschi. The Americas have their own genus: Sesarma. But Pseudosesarma are often in pet shops in Europe and in the US too. They are social animals which should be kept in groups – but wisely in harem system (more females than males). The males are very territorial and may fight so there should be enough space and hiding places.
They can be held with brackish water but either with true freshwater. Just their larvae need saltwater again – but You cannot breed in the keeping tank anyway. It's a little bit of work to breed those species but possible.
Do you know already how big Your tank can or shall be?
Oh, before I got: There are some marine species either that live in estuaries and can even live in freshwater as Scylla serrata or some Callinectes species - members of the mentioned swimming crabs, family Portunidae. You probably won't fight those species in the pet trade, more likely on a fish market.
