However, I'm fairly certain in the regs it mentions that you are not supposed to fly beyond gliding distance of land without a life presever. Haven't looked it up, so can't give the exact reg.
Nope, not for Part 91 (non-commercial)light aircraft operations in the U.S. Entirely up to the pilot or aircraft operator.
You're right the gliding distance is a 135 (Air Taxi) reg, and for 91 you only need them when flying further than 50nm from shore in a large(12,500 lbs +) or turbine powered airplane.
Edit: Of course I think staying within gliding distance is a great idea when you don't have proper flotation equipment, but with the low weather it may not have been possible that day.