You couldn't be more Dam wrong.
Mind, where it does work, it's phenomenally effective, efficient, and responsive.
There are a few sites at which seawater-based systems might be possible, in which the ocean forms the "lower reservoir". These are dependent on suitable terrain. Matching terrain to consumption patterns is difficult: the Netherlands and much of Britain are sorely lacking. Some of the best potential sites are along the Balkan coast in Serbia and Croatia. Chile's Atacama Desert, along the Pacific coastline, is nearly ideal geographically, but is far from most use (North America, Europe, Asia). Portions of the US West Coast might be suitable, though would all but certainly face major political resistance for environmental impacts.
And: working with seawater is complex from an engineering standpoint: it's corrosive and sea life has a pronounced tendency to foul large-scale water-handling systems, though this may be tractable. There've been several pilot projects, though those have since been decomissioned, excepting Rance in France, designed as a tidal power plant, though capable of working as a pumped-hydro facility.
Terrible energy density, massive land use.
To store the energy contained in 1 gallon of gasoline requires over 55,000 gallons to be pumped up 726 feet (CCST 2012). [1]
Itaipu Dam has 14,000 MW installed power, 1,350 square kilometres (520 sq mi) were flooded. [2]
[1] http://energyskeptic.com/2015/hydropower-has-a-very-low-ener...
Hydroelectric projects also virtually always provide additional services such as flood control, irrigation water, recreation, and waterways management (ensuring water flows, etc.), which also account for design elements including scale, etc.
Pumped hydro should provide a sufficient level of both capacity and storage, though in general the storage requirements are far smaller than traditional hydroelectric dams aimed at generation. There are quite small pumped hydro facilities, the compensation being that water levels can rise or fall considerably, often many metres in a single day's storage/generation cycle.
Gravity is in general a weak force, but water is relatively massive, largely non-toxic, and can be utilised through large-scale pump-generator units (~800 MW per generator IIRC), which is a scale few other options can match. Again, on balance, pumped hydro is a good solution, there's just not enough of it to go around.
Converting to sensible units gives
mass = 55e3 * 3.79kg/gallon ~ 208450kg
height = 221 meters
mass * height * 9.8m/s^2 ~ 452043210 joules (watt-seconds)
or 125KwH, 70% of that is 87KwH. What does gasoline have to do with energy storage on the grid? Furthermore, a gallon of gas has 33KwH [2].
What does the Itaipu Dam's installed power have to do with stored energy?
[1] https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...
[2] https://en.wikipedia.org/wiki/Gasoline_gallon_equivalent
How many sites do we need? Why would the Netherlands need pumped hydro, isn't the European grid interconnected?
See my comment below.
https://www.energy.gov/sites/prod/files/2018/04/f51/Hydropow...
Likewise, you seem to have failed to grasp what I'd hoped would be a rather evident reason for mentioning the Netherlands in the context of pumped hydro and terrain.
