> The rationale is that sea landings make repair work much harder.

Ist the much more importend things the time and cost of getting the rocket back to where you want it much bigger?

I imagine that's a factor, but it's primarily the fact that sea water is really hard on the delicate parts of the rocket due to corrosion and mineral deposits.

And very rapid cooling as well as the chance of impact damage. Sea landings may look gentle but really they aren't.

There was an SSTO proposal from the 1970's that did involve building a huge splashdown lake. That proposal also used vertical landing, however. (The craft was so large, landing gear would've been prohibitive, so the whole thing just floated.)

Mars does not currently have lakes.