Our mentor the Pond Boss Bob Lusk has a zip line and no island. An island will make aerating and mixing the pond much more difficult due to the baffle affect restricting water circulation. An island in 2 acres requires more diffusers and probably a second compressor. Plan on the pond having water clarity of 2-3 ft. It can be difficult to maintain average water clarity deeper than that. A lot of clarity will depend on water shed and and ability to keep productivity low and type of fishery present. Get a consultation and second opinions from Lusk and Mike Otto before moving dirt.