How should I go about fullfilling the Splashdown! contract please?

@Toinkove ya man, I launched from DSC.
The waypoint reaches to the ocean floor so it can be done with anything that can crawl on the bottom.

I drove there from a launch pad with a wheeled rover.. under water too...
Yeah...

If it’s the one where you land in DSC Bay, I used a poorly engineered rocket. It took several attempts, and I’d recommend a plane that lands by parachute that is flown by a rocket.