Unless its a record early ice out, this time of year I can probably park my truck just about anywhere I want more easily than I can anchor my boat, although neither seems like a good idea the moment. I've been on the other side of inconsiderate boaters anchoring in navigation lanes or not securing their boats, as well as seen divers without flags under Long Island bridge on a weekend in July and hope to never join that crowd. The beauty of anchoring to dive is that I can check that your anchor is properly set before I leave the line, so the boat should be there when I get back.
I don't Ice Dive, I'm a fairly new diver and have done one dive that was a fair amount colder than I expected, 43° at Nubble light. That convinced me that I have no interest in getting ice diving certified anytime soon, I'll stick to well above 45° water. I'm just looking forward to getting out on Winnipesaukee this summer once it warms up enough.
As for sitting in the boat I actually have a friend who doesn't dive but likes to come out on the water, but won't have that every time which is why I was asking if there was any truth to the rumors I'd heard about not having un-attended boats anchored on the lake.
I'm planning to take a night diving class this spring, but in order to avoid getting run over and/or harassed by the MP at night it seems that it would make sense to light your dive flag on your boat, as well as use the lights shown here instead of an anchor light
http://www.navcen.uscg.gov/?pageName=Rule27 in section E. Although its not listed in the NH boating laws anywhere so I'm not sure if the MP or anyone on the lake would know what to make of it.