Which ocean are you near? (I am assuming it is an ocean beach and not one of the Great Lakes or some other large body of water.) It reminds me of something, but I can't say for sure without knowing the general location of the find.
It's pretty thin and very light. I showed it to a few people and they think it's from the English civil war period. I should say that I found it on a beach in Ireland. I will take a picture of the reverse side but it looks like the symbol was hammered in.