Usually the open water and thinner ice in conjunction with the diffuser is very localized but never take that for granted. A lot of it depends on the temperatures, and the wind when the ice was forming, which in tandem with the diffuser bubble can slow down ice formation a fair distance away.
Short answer: I think you'll be O.K and have safe ice adjacent to the dam but it always a good idea to check with an auger or spud.
