Another vote for cooking them in the water they came out of.
My method is to pull the pot, whack the keepers with a crescent wrench just above the point of their genital flap to stun/kill them, rip the shell off and keep it for the fish checkers, grab the legs and fold/rip the body in half, shake out the goo, and remove the gills. Then I throw them in a 5 gal bucket of water I’ve collected for transport home. This is the water they’ll be steamed in.
It’s probably bullshit because they’re dead and I’m steaming them so they aren’t actually in the water, but I learned this from my grandpa who claimed the reason is that it’s salinity their flesh is used to. He boiled them, I steam them, but the theory (superstition?) is that saltier water will mean the flesh absorbs more salt to try and equalize salinity in and outside of cells, while lower salinity means they will expel some of their delicious saltiness to try and equalize. Plus, it’s easier than mixing your own. Having written this all out, this is probably the main reason grandpa did it, and I’m just following tradition.
Edit: Forgot a sentence.