Welcome to New England.
As you can see from all the responses, this is a hard question to answer. I see some people answering 3mm for New England diving......but I think that is more the exception that the rule. If you are diving shallow in later summer in southern New England....maybe. In areas north of Cape Cod, bottom temps at deeper depths (80-100) will be in the 40s at their WARMEST. North of of Boston, water temps (not counting surface temps) never get much above 60. In RI and southern MA , the upper limit is closer to 70. In NH and into ME -- low 50s are the highest.
You want to give yourself flexibility, even within New England. As a result, you want to choose something that allows you to cover multiple bases. Part of it depends on personal preference and comfort level; part of it depends on the type of diving you will be doing -- location, depth, repetitive (2-3) diving, boat/shore, etc.
As for HI -- another personal choice issue. I know folks that have brought their drysuits to HI (shell suits with minimal exposure protection underneath)....and per their comments, they were warm and happy on the dive boat while others were chilly by the end of the day. Others swear by a 3mm shorty in low 70sF water......
I tend to think you should slightly over-protect yourself -- I can't say that I've ever been that hot while underwater. I've dove my 7mm farmer john/jacket in the St. Lawrence river in 70F water (no hood and tropical gloves) and was completely comfortable. Others would call diving with 7mm in that kind of temp crazy. I own a drysuit and dive that in NE almost exclusively. I also have a 7mm wetsuit (largely retired) and a 3mm full (Caribbean). I've dove the 3mm in 82-84 water....and again, I was completely comfortable.
You mileage may vary ;-)
Chris