How close you should fish to islands all depends on the fish.
F1s like around 18ins of water and I normally catch them by casting just short of the island.
Carp prefer 12ins of depth and can be attacked by chucking tight. Late in the day the carp may well end up almost hugging the island, moving into the really shallow water.
To find this depth, cast out with a bomb and try to get a feel of things. You won’t have a count, as the water is too shallow, but if it’s a foot deep, the lead should hit bottom immediately. If it’s a bit deeper, it’ll take a fraction longer to settle.
Carp prefer 12ins of depth near islands
Carp prefer 12ins of depth near islands