the sea crab and the ocean crab are two different crabs apparently. the ocean crab is the one that drops the bronze box. the sea crab is the one that spawns at the beginning of each mahura boat ride. to fish up the ocean crab, i used a clothespole and lugworms
hmm...i guess on this site it says the crab DOES drop the box but i got the drop from teh ocean crab the first time. i've killed a billion of the sea crabs with no box drops