Info from Brooklyn Bridge Park website:

Grab your towel and shades for swimming, sunning, lounging, picnicing and playing in Brooklyn Bridge Park this summer.

Cost: FREE!

Hours:
Pool – 11 a.m. – 7 p.m.
Beach – 9 a.m. – 7 p.m.
7 days a week

What’s there to do? Take a dip in the 25 meter, 7 lane Floating Pool Lady,
a floating swimming pool moored in the East River. Run your toes
through the sand on the 40,000 square foot Brooklyn Bridge Park Beach.
Grab a burger, rent an umbrella and enjoy the spectacular views of New
York Harbor. Get your game on for pick-up beach volleyball and sand
soccer or contact MetroSoccer to sign up for leagues and clinics!

Where: In the future Brooklyn Bridge Park! Between
Piers 4 and 5 (Furman and Joralemon Streets), one block north of
Atlantic Avenue. Mapquest: 334 Furman Street.

How do I get there?