They are smallish, about the size of a call duck, with pointy yellow bills and dark legs. I've seen them standing on the bank and they are very white from the waterline down; some white shows at the end of the wing when they float and when they flap.
They are very busy diving and foraging the pond: lots of frogs and crayfish, but no fish here. The pond is man-made, spring-fed and about 50'x80'. Quite low at this time.
I have not seen them fly, although they've appeared to be gone a few times. I'm between Conway Lake and the Saco River, so they have lots of options.
Today, it's just party of one, which I hope is temporary. We did have some big weather night before last, with quite a few trees and limbs down. I'd like to think he/she just didn't feel like going out with the others.
Anyway, if they are red-necked grebes, I believe that would make them unusual for Conway, NH.