There are some beaches but they're not like east coast beaches at all, and it will be too early to swim. But if you just want to walk along the water, I like Magnuson Park on Lake Washington, and Kayak Point north of Everett, which is on the Puget Sound. On the drive over, there is also Lake Chelan which will be COLD but is a beautiful part of the state, and not that far from the Grand Coulee Dam.