TLDR: Sank to the bottom of the ocean for science!
I spent most of yesterday trying to get the "landed on water" science. I had read that the two ways to do it are to find a tiny island in the water biome to land on, or else reach the bottom of the ocean in a heavier than water rover. I downloaded the Field Research mod which gave me waypoints in a contract to some island suggestions, but they were all near the poles. I figured that it would take too long to get there by plane, and that the islands would be too small to hit by rocket. So I started trying to design a heavier than water rover. After several hours and even more failed prototypes, I finally got one that could travel underwater but it still had problems. The rate of travel was SOOO slow (0.3m/s-1.1m/s) and used up so much power that I couldn't make it past the shores biome. Then I had an idea... I could build a heavier than water craft with none of the frills like wheels or batteries or solar panels (which are all apparently buoyant), use a rocket engine to get it over the water biome, then just let it sink! It took only a few tries to get the design nailed down, but this ended up working out way better than my stupid rover designs.