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.

@CloakPin DAMN IT THAT WORKED!!! I'm assuming you had DSC pad unlocked first. I had considered doing the same with my plane (didn't have DSC anything yet) after it couldn't set down gently enough but decided there was no way that was gonna pass. Wish I had now!

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

I think the game intended you to get accesses to a second control surface for wings, then build a plane with alerions & flaps so you can gently set it down on the surface under 20 m/s. Without flaps I managed 34 m/s at best! so with flaps I think it would be very doable. But I wasn't into the planes stuff so I built the tiny rocket and lander bellow that works for this as well.

https://www.simplerockets.com/c/P4P8a9/Bebop-Lander

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.