Last weekend we went out to the saltwater beach at Golden Gardens (pictured below). The saltwater is the Puget Sound, which is super deep and SUPER COLD (55F).
We like to get our feet wet while we're there, then warm them up in the sand and go play on the amazing new playground or go look at the tidepools if the tide is out (something I have only realized since moving to Seattle - not many tidepools in Iowa!). The nice thing about this beach is that it's not so big that you can't find your friends (and we often do, even when we didn't plan to), but it's still big enough to find lots to do and see. You can have bonfires, buy ice cream, set sail on a boat, play in a stream, watch trains go by, grill out or have a party, play frisbee or volleyball, or watch people doing any of the above (which is what we usually do).
We also had a 2.5-yr birthday party for the kids at a public pool - we rented it out for the 'party' but really we just wanted to have the pool to ourselves since it's normally really really crowded (see below).
The pool has a giant waterslide that we went down about 800 times. It also has a really warm kid's pool with a smaller water slide. It's a little hazardous swimming with a bunch of 2.5-yr olds - they're adventurous enough to try to get away from you but not smart enough to know what's really dangerous. Many of us parents had a few moments of terror when our kids (briefly) got away from us at various points in the swim. Fortunately Layla was pretty good. She didn't want to do the waterslide by herself and listened when we told her not to run. She LOVES the water and has no fear of putting her face in or getting splashed. She also really had a lot of fun playing with all her friends.