Complex sentence: The Coast Guard first tried to take them on board via an inflatable boat, but after it was damaged when trying to approach the "Satori" they sent a helicopter, which is a much riskier approach as a rescue swimmer must jump in the dangerous seas.
Split into simpler sentences: The Coast Guard first tried to take them on board via an inflatable boat. Only after the inflatable boat was damaged when trying to approach the "Satori" did the Coast Guard send a helicopter, which is a much riskier approach as a rescue swimmer must jump in the dangerous seas.