For surf perch, I use a clam net hooked to my belt. Drop fish in, it stays wet in the surf because I'm knee deep. Some keep a 5 gallon bucket with seawater up on the beach but that keeps you from moving and I've had surf reach it and knock it over.
Then into a kill bag with a bag of ice for the trip home after cleaning, like all my fish. Crab you put in a bucket without water, and a wet burlap bag over the top. Again when its time to leave, clean them and pop 'em into the kill bag.