I live in Ross, Skye and Lochaber constituency, which, if you look carefully, has a large island involved! So do a number of other constituencies, including I O Sheppey off Kent. Your point is invalid.
Skye has nowhere near enough people to be one seat, while the Isle of Sheppey is barely an island and has road links with the mainland. The Isle of Wight doesn't have this. It's in a position where certainly it is at least entitled to one seat on its own. If the population grows too big for one, two seats isn't "unsustainable". There will be pitchforks and wickermen at the thought of "Ryde and Portsmouth Harbour"
Ahem, Southsea and Ryde BC if you please...
Seriously we did do some preliminary work on this prior to the amendment to the legislation going through. Somewhere on the boundaries thread is an IoW/Hampshire plan I did that had Southsea and Ryde.
Interestingly though, and rather more predictably, the papers released from the time before the amendment was put through (I think the link was posted on here by David) points to the parts to be connected to would be the New Forest and western Wight on the quite sensible reasoning that rural west Wight and rural New Forest have far mor in common then either of the two other alternatives.