If your idea of a great vacation means being by the water, make a beeline for Yerba Buena Island. Whatever you choose to do here, you're unlikely to get bored of those pretty views.
If you're going to Yerba Buena Island in Treasure Island, make time for other places too. After you've ticked off this water-surrounded land mass, Pier 39 is close by and will keep you going for a bit longer.