My father grew up in Sherwood Forest (no really!), up the Severn river from Annapolis and the US Naval Academy, and, until I was in middle school, had a small sail boat he would take us out on for the weekend (it was small but you could sleep in the bow). In those days we would take handful of raw chicken parts, tie them to string, and then catch crabs (Chesapeake blue crabs) off the side of the boat. You would go from line to line and check to see if you could feel one nibbling, and if so, very slowly pull the line up towards the surface so you could net the crab. That method is called crabbing with a handline, and you can do that in Maryland without a license as long as you don't catch any more than 2 dozen crabs in a day.
Nowadays my pops has a house on the other side of the Chesapeake Bay (on the Choptank river) and has a boat with a recreational crabbing license which allows him to set out a trotline. A trotline is just a long line with bait (chicken necks typically) tied onto it every couple of feet. It is anchored at both ends, lays on the bottom of the river, and has floats that mark each end. He has two spools, each 500' long, that he links together. These get fed out over the y shaped piece of pipe that you can see in the picture above. There is, of course, a lot of art as to when and where to place the line... whether or not the tide is coming in and all sorts of stuff I don't really understand. In our particular case it was a windy and gray day, so my dad chose a more sheltered area with fewer crabs but where the wind wouldn't be a huge hassle.
Once the line is all laid out you take the boat back up to the start of the line (giving the crabs some time to find the bait) and feed it over another set of pipe that will bring the line up towards the surface as we drive the boat down its length. As long as you go slowly enough the crabs will be so intent on their chicken dinner that they won't sense that they are moving towards the surface.
Now, the guy driving the boat can also net the crabs (my dad did this on the first pass and it looked pretty hard), but it's a lot easier to have somebody else to it. The officially recommended way to hold the net is not how Anna has it above... you want your right hand at the top and the pole in front of your body... but for whatever reason it seems the natural instinct is the opposite. As the line is brought up to the surface you keep your eye out for crabs clinging to the bait and net any you see with a side to side action (as opposed to up and down) and dump them into a bushel basket. It's pretty fun actually... though I suppose less so for the crabs.
At the end of the line you head back up to the beginning so you can start again, but on the way you need to check to see which ones you netted were keepers. So you grab them with tongs (not hands! those claws hurt) and compare them to the above handy dandy crab measuring calipers... the notch higher up is 5" for early in the season and the one at the bottom is 5 and 1/4" for latter. If they are too small you throw them back and if they are keepers they go into a second bushel basket. In addition, recreational crabbers like us can only take males, so all females get thrown back. How can you tell? It's as easy as distinguishing the Capitol from the Washington Monument.
The guy above appears to have lost a fight and one of his claws with it. Crabs are pretty ornery... as you might have heard. You can see why they are called blue crabs though... the amount of green in the shell varies a lot it seems, but they always have a lot of blue in their limbs.
Back at shore after a successful couple of hours (though not especially so... we only caught about a dozen, whereas my dad says on a good day he could come back with a full bushel in that time) we prepped them to cook. Couldn't be simpler... put them in a steamer basket and dump a bunch of Old Bay and salt on them... then steam them over a mixture of water and vinegar.
Additional photos of the excursion can be seen on Flickr.