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