Shallow Water
It's best to choose shallow water where your trap or net can reach the bottom of the river. Crabs are active in the warm months as they spawn. If your crab trap can't reach the bottom of the water, it won't reach the mucky rocks and scum where the crabs like to live. If you are fishing from a boat, stay close to shore in waters shallow enough for your nets to reach the bottom.
Brackish Water
Crabs can live in fresh or salt water, but prefer brackish water like that found in marshes, in the New York Bay and up the tidal estuary section of the river, accessible from cities and parks in New York and New Jersey. Crabs are more active during moving tides, so placing your nets in the estuary during a changing tide may increase your catch. If you want to crab in the tidal estuary, but don't want to deal with crowds, try accessing the Hudson River from the New Jersey section of the Palisades Interstate Park, which allows fishing and crabbing on the New Jersey side of the river.
Near Walls and Piers
In addition to providing access for crabbers on foot, walls and piers create a sheltered habitat for the crabs. You can even fish with a fishing line and net for crabs from piers, lowering a baited line into the water and lifting the crab into a net.