1. "Billy the Squid," by Tom Chapin and John Forster ("He robbed from the selfish and gave to the shellfish"). Great sing.

2. "The Geoduck Song," as sung by Fred Penner. It's a kind of clam and it's pronounced Gooeyduck and it's a lot of fun to sing.
Dig a duck, dig a duck, dig a gooeyduck
Dig a duck, dig a gooeyduyck, dig a duck a day...

3. "Duck for the Oyster, Dive for the Clam." It's a square dance we used to do in elementary school.

And, of course, any song by Shell Silverstein