Complex sentence: He put his training and certifications as a SCUBA diver in the Neutral Buoyancy Laboratory assisting with training dives with astronauts with underwater mockups of the International Space Station.
Split into simpler sentences: He put his training and certifications as a SCUBA diver to use in the Neutral Buoyancy Laboratory. There he assisted with training dives with astronauts with underwater mockups of the International Space Station.