Actually, this scheme is currently employed by utilities, albeit usually not with seawater. The technique is called pumped storage hydro. Pumped storage hydro accounts for the vast majority of grid energy storage world-wide. Pumped storage hydro is used to by power companies to achieve various goals, e.g.:
flatten out load variations (as you suggested elsewhere in this thread)
provide “instant-on” reserve generation for voltage and frequency support
level out the fluctuating output of intermittent energy sources such as wind and solar (as you suggested above)
Wikipedia states that round-trip efficiency of pumped storage hydro can range between 70% and 87%, making it an economical solution in many cases.
A couple of obstacles to using pumped storage hydro are:
Certain topological/geographic features are needed to make PSH viable
Social and ecological concerns
I have taken the survey.