LEO let him go because he's IN the water. They have no jurisdiction IN the water. (One should ask HOW he got to that location? By boat, or by beach?)
All waters surrounding the USA fall under diff rules than the Land of the USA.
That being said, his breach of Land-to-water could have been an issue upon coming ashore. I'm guessing there was some reciprocal respect / professional courtesy given.
Thank about the DRY FOOT rules for Illegals trying to get here from foreign countries. They don't fall under our laws until they come on land.