Turkeys love to roost over or right beside the water. There is a good chance that they will fly down onto the island (depending on how big it is) and then fly over to mainland mid-morning. Then, as evening progresses, there is a fair chance that they'll fly back over before flying up to the roost. Although, if I was a turkey, I'd just fly straight to the roost from the mainland. When you are dealing with turkeys (especially easterns, generally) you really have to pattern them well when water is involved. Usually the birds will have a set course that they walk everyday, and usually they will even roost in nearly the same places. If you can find where the birds are traveling and where they are crossing the watercourse, all you need to do is beat them there in the morning.