I am not fair skinned but past experience has taught me I burn relatively quickly.
In 2011 I took part in IM Florida. So, same time of year and albeit diagonally opposite corner of the State, pretty similar weather conditions I imagine.
I took advice from these kind pirates and invested in P20.
The factor 20 (you can get a lot higher) went on liberally within 10 minutes of me getting up on race day morning and again for good measure ten minutes before I left the apartment (well before wettie went on).
The plastic number they stuck (or tried to stick) to my leg lasted seconds. The sun cream lasted 12 hours 51 minutes. Although to be fair, the sun probably set a couple of hours before I crossed the finish line.