Hallaheli,
It's been six or seven years since I flew a "straight A" model S-76 in the Gulf of Mexico. However, in the summertime the ambient conditions are often as you have stated. A typical trip then was 75-100 miles offshore.
We were required to carry sufficient fuel to complete the trip and return to a suitable landing site on the beach with 30 minutes reserve (no one-way fuel, as the capability to make a single-engine landing offshore was rather marginal). We operated under category "B" only.
With a 20 year old helicopter configured and equipped for offshore operations, I recall, the payload (pax, bags, and cargo) under those conditions was typically limited to approximately 1500#, and that figure is probably generous.
-Stan-