I think, it could be nice to add trap for crab. You could build trap from metal and ropes. Throw trap to water with bait (like fish, potato) and some time later pick trap with crab. Second suggestion is add night .
The crab traps and bait sounds awesome. Perhaps you could craft different types of bait from different resources. (i.e. potato and fish or potato and shark) And different bait types will have a higher chance of attracting more crabs. Perhaps a type of bait could be made that attracts a rare crab type.