Marine organisms need to be kept in sea water and will die if put into freshwater.
Marine crabs are also predators that will eat anything they can catch. As a general rule, crabs should not be kept with fish because the fish get eaten.
Absolutely not - never put an animal you find in a saltwater environment into freshwater.
While there are a some organisms that can gradually transition between the two environments, crabs are not one of them. Coastal crab species can sometimes tolerate less than full seawater salinity and may be found in environments that are closer to brackish like estuaries, but they do not venture into full freshwater and will die if exposed to it for too long. This is true of both marine true crabs and marine hermit crabs. Please leave the crabs on the beach unless you already have a coldwater saltwater aquarium set up that is also suitable for the species in question.
On a related note, there is at least one species of truly freshwater true crab that has made an appearance in the hobby (Limnopilos naiyanetri), but it is very small - to the point of being likely to be eaten by fish. It is also still quite rare in the pet trade. There are also some marine true crabs that do just fine with fish in marine aquariums, but they are not typically found in tidal environments. Most coastal species that you'd be likely to find on a beach in the UK are extremely aggressive in an aquarium setting and require species tanks.