I hear the Long Island Sound waters can be cold well into summer. An recos for swimming beaches?

There's a great beach in Mamaroneck- Harbor Island Beach. It's a small but fun beach and it's usually not too crowded. If you want a bigger beach head to Rye Playland beach where you will find a ton of activities throughout the summer including volleyball and music entertainment.

PS If you have a dog that loves swimming, check out Larchmont's Dog Park; http://larchmont.patch.com/articles/making-a-splash-on-the-dog-days-of-summer