§121.615 Dispatch or flight release over water: Flag and supplemental operations.
(a) No person may dispatch or release an aircraft for a flight that involves extended overwater operation unless appropriate weather reports or forecasts or any combination thereof, indicate that the weather conditions will be at or above the authorized minimums at the estimated time of arrival at any airport to which dispatched or released or to any required alternate airport.
You appear to already have this, but here is the link anyway:
http://ecfrback.access.gpo.gov/otcgi...Sec.%20121.615
My Ops Manual discusses this under the paragraph "Extended Overwater Dispatch." It says," A flight may be dispatched for overwater operations to a destination forecast below landing minimums provided the filed alternate airport meets alternate weather requirements." This is reinforced during our annual International Ops Specs training.
This appears to be standard for all scheduled US Flag Ops manuals.