We have friends who have a family rule- two pilots two engines only. For us we prefer SBC over WinAir but we now use Tradewind thru SJU.
Interesting to see that frequent travelers don't always look for the best prices, but consider safety too.
Winair
Aircraft type: De Havilland Twin Otter - Twin engine turbo prop
Pilots: 2
Regulating authority: CMCAA (St. Maarten Civil Aviation Authority) - SINT MAARTEN
Hub: SXM
St Barth Commuter
Aircraft type: Cessna Caravan - Single engine turbo prop
Pilot: 1 (no exception, even on private charters)
Regulating authority: DGAC (Direction Générale de l'Aviation Civile), EASA (European Aviation Safety Agency) - FRANCE
Hub: SBH
Tradewind Aviation
Aircraft type: Pilatus PC 12 - Single engine turbo prop
Pilots: 2
Regulating authority: FAA (Federal Aviation Administration) - USA
Hub: SJU
Air Safety is definitely about the number of engines (particularly when the flight is 99% over water), number of pilots (what if the Captain gets a heart attack during flight?), but also about the regulating authority and their standards. There is more involved, such as aircraft maintenance programs, fleet age, crew training (SBH is a special airport), ....
About flying over water on a single engine aircraft:
Operators have to demonstrate to the authority that in case of engine failure, the airplane will be able to "glide" without engine to the nearest land. "Altitude" is often the trade off: the higher the airplane, the longer the gliding distance. This is particularly important when considering single-engine between St Barth and San Juan, where the stretch of water is close to 90 miles without any land in between (Virgin islands to St. Maarten). As opposed to SXM SBH, where it is only 10 miles.
Some agencies, such as ARG/US and Wyvern, also use their own safety directives to assess air operators.
More info:
safety standards.