Touching down on water for the purpose of shortening a landing, gravel bars, beaches etc in some parts of the world is no doubt a requirement/necessity.
Whether a rescue operation or convenience or just "handy".
It is obviously relatively safe, though not as safe as a nice flat strip of grass.
If pilots are operating in such situations then practice would be required, certainly a sensible thing to do prior to a first attempt out of reach of help when/if it goes belly up.
There are plenty of places in Scotland among lochs and shallow rivers where suitable flat ground is not available in the event of a problem, weather or engine etc.
I don't see this practice as any more dangerous than practicing stalls or spin recovery, in the right kind of aircraft.
I also don't think an aerobatic course to teach accuracy of flying would help.