Went out there a couple weeks ago. The big issue is parking. Now that it's past Memorial Day, I think you'd have to pay $20 to park at Good Harbor. If you park further up north, which is closer to the island entry point anyhow, you need a resident sticker and the motel is very vigilant about towing. If you're going to park in Good Harbor, drive one exit past on the right and ditch your stuff by the motel.
At low tide, you can actually walk right up to the island -- you can get in with a short surface swim even if you're not at slack tide.
I have been really disappointed with the two dives I did there. On the first one, it was really our fault -- we assumed the island was not that big around and unwisely decided to circumnavigate it in soupy fog. The second time around, I did not enjoy the strong surge next to the island, vis was pretty poor due to the stirred up silt, and I have found a lot more lobsters at other sites. Not sure why it has gotten a reputation for good lobstering.