I have used a brand called Bullfrog when I will be swimming or otherwise doing something active that may cause normal sunscreen to be washed away or sweated off. The gel-type seems to last very well and comes in higher SPF numbers. I would make sure to find some that says sport or for swimming, if you plan to be in the water.
The other posters made good points too. I am very fair skinned and tend to burn very quickly, so I know that a sunburn can seriously ruin a vacation