Earlier quoted context omitted.
Are you just planning to camp or do you plan to setup a small cabin? Can you dig a well there? Will you allow guests to visit? A tiny produce garden? Helipad? ...I have so many, I can't type fast enough!
Sweetwater well on a saltwater island this size is a rare thing.
Not at all. It's not uncommon to have a fresh water table beneath the surface of an area dotted with islands. If you doubt this, ask yourself where the tree's roots go, and how those salt-intolerant trees survive.
I live 100 feet from salt water in Washington State and I have a perfectly reliable fresh-water well.