San Diego is home to the beautiful Mission Bay Park- at 4,235 acres is the largest aquatic park in the U.S. There's a lot of water fun to be had here!
Whether you enjoy kayaking, wind surfing (pictured here), paddle boarding, fishing, scuba diving, snorkeling, jet skiing, jet pack, etc. There's also trails and park areas for landlubbers.
You can find several rental places along the bay for whatever activity you would like to try. The Mission Bay Aquatic Center is a good place to start- the link is included below.