You can reach the Sainte Marguerite island with your own ship... or via public ferry service e.g. from Nice, Cannes or La Napoule.
This multilevel cache, which consists out of two stages, should be solved comfortable within 2 hours (excl. shipping times).
We recommend to take at summer time enough water, your self composed picnic and swimming equipment with you to assure the highest possible enjoyment level for you. [:-)]
One possible starting point is in Cannes at N 43° 32.802', E 007° 00.831'
Have fun and good luck.
Note on cache maintenance: I live at the same time in the south of french and in germany, so the maintenance of caches at these two places is no problem.