That's a total range of 7F. Compared with the much, much larger range of temperatures here locally (surface water temperatures from 80F down to cold enough to drive a truck on), 7F is hardly any change at all - it's about 1.8% in terms of degree change.*
7f., when it's from 84+-77+, is a very big difference in diving comfort. To compare your local temps to the temps here is really irrelevent. Diving in water that is under 80f. can get really cold no matter what the percentage of difference there is. When I dive in the winter for a few days in a row, even with full wetsuit, vest and hood, my whole body will shudder from the chill while diving. In the winter it is not uncommon for people who dive many days in a row to surface early because they are cold. As I mentioned in my other post you can see the fairly extreme difference in diving apparel. As far as diving without a wetsuit...I take out divers from the Northern states that are used to really cold weather and don't wear wetsuits, but these guys are rare and usually have a lot extra body mass. A person living in New Hampshire, like yourself, is much closer to Quebec than you are the Tropics so 77-78f water may seem warm for you, but for most divers expecially women, kids and slender folks.....that water is going to seem really cold after a couple of days. People in the tropics have much thinner blood and chill much easier, but in general during the winter almost everyone wears a wetsuit. Look at the pix of Cozumel DM's during the winter.....almost all are head to toe in neoprene and wearing sweatshirts on-board. You are not doing anyone any favors by implying that thermal protection is not needed.
Starting about this time of year it is a good idea to take a sweatshirt on the boat as the winter is usually breezier (sp?) and the lower air temps can make it really chilly during your SI.
