Yes, the water shoes come in very handy if you do any of the beach or snorkel excursions. Even at beautiful Motu Mahana (the free private motu picnic day at Taha'a) there are sea urchins and the possibility of cone shells in the lagoon and it is important to have your feet protected. FP has very few sandy beaches (the longest stretch of nice sand we experienced was on the Bora Bora motu) so you'll find good use for your watershoes.
