In muddier sandy areas, dig down to about a foot or so for steamers - they and horse clams are better for chowder IMHO. I stay up high in the loose gravels on Hood Canal and use a dead/dull oyster shell to just push gravel aside for Manilla clams which tend to be smaller and more tender for pasta or just general eating . Once I find a good spot, I can usually get a limit within about 15 minutes and never get wet at all. I look for areas where the wave action caused the gravels to pile up in ridges during the last big blow. Many times live clams are right on, or just under the surface.
WDFW webpage has a marine biotoxin webpage link with the latest beach closures. Somewhere in those pages there are also maps of public beaches good for clamming/oysters.
Have fun!