Floating solar seems to make good sense; the 'land' is otherwise unused and unusable, the water can be used for cooling, it offers shade and a structure for marine life. Of course cabling and storm-proofing etc means much more expense, but if we're talking about gigantic installations ...
They could function as naval superchargers for ships.
Certain places are better for this than others. Offshore of Rhodes, eastern South Korea, Monaco, the Philippines have very deep water not far off shore.
Tides have massive potential for power generation. The best part is how predictable they are.
Your idea of weights being raised and lowered makes more sense for land based energy storage (and it is being trialed right now, with promising results).