We have finally found a surf bikini that stays on when the surf is pumping. Honey Girl makes bikini bottoms with a string at the waistband so that even when you get caught inside by a monster, your suit stays on. They made this bikinis for surfing. Honey girl has styles with more or less coverage so that you can choose your tan line.
Most girls here live in their suits, so make sure to bring an extra or two to wear when one is wet from the surf session.