I know people DO dive wet in the Sound, but I can't imagine doing it, and I especially can't imagine any enthusiasm for a multi-dive day later in the year than -- maybe -- October.
I have both a trilam and a compressed neoprene suit, and each has its virtues. The trilam is lighter to haul around and dries a little faster, and doesn't have to fit as well. The neoprene is MUCH warmer and has some stretch to it, but it's heavy to lug around (not a problem while wearing it). It's also bulkier by quite a bit, so I have to readjust my harness to go from one suit to the other.
As others have said, it's not expensive to buy a 3 mil suit for the tropics. I know SB's kidspot dove dry for a while on Maui and liked it (and there's still the attraction of peeling off the suit and being warm and DRY), but I've never been willing to make the effort to wad up a drysuit and try to pack it in my luggage. Wetsuits are much more obliging that way.
The other advantage to a wetsuit is that they aren't as fragile -- get a small rip in one, you're still likely to go diving, whereas if you tear a seal, your diving day may be finished. There are ways around that (eg. zipseals), but Bob's saying is still a truism: "All dry suits become wetsuits, and then you get them fixed."
You can get good buys on drysuits if you're patient and diligent. I got my first one for $800 because the shop wasn't carrying the brand any more. Penopolypants just posted that Underwater Sports is apparently offering a new USIA suit for something like $975. My husband got a used High Tide (over $2000 new) for $600, plus what it cost to put new boots on it. Don't write a dry suit off over cost.