Although deuterium can only be found in 1 part per 6000 molecules, the energy released is sufficient for their needs without necessary calling on their reserve supplies. Deuterium's nuclear fusion, combined with using the remaining water as a high-powered thruster propulsion propellant combined with both Thunderbirds' ramjets. Most hypersonic flight would also be over water than land as well. All the Thunderbird vehicles are equipped with nuclear reactors but it would be unwise to exhaust nuclear fuel lest it contaminates the world as they fly past. This solves a number of problems, especially at take-off.
Ramjets are only effective at high velocity when they are drawing oxygen on board.
Had Thunderbird One been using a high octane fuel, its launch bay window could not possibly contain all its exhaust. The patio and house would continually smell and be stained by the fuel. A dead giveaway to any visitors of some unusual aircraft in the area. Using water, it would appear that the island had had a brief shower and with tropical temperatures be quickly evaporated. Thunderbird Two, when leaving the island, needs the ramp for take-off.