I will admit I have not heard of Casimir force/effect. From gleaning an article it appears that this would have some effect with micro-machines. Perhaps as technology focuses on smaller devices Casimir will be more relevant.
The force could affect everything from micromachines to unified theories of nature
physicsworld.com
The cassimir effect works in a reversed log effect too though, check out john trumps work in this log effect. Pump 3 million volts through two plates and you can generate 5000lbs of force. But in the cassimir effect if you apply force the output effect goes up logarithmically in response. It's kinda like magnets, couple of small ones takes effort but a human can push them together, double the size though and it would take 5 humans to push them together...or something, using arbitrary numbers here.
One solid pipe of titanium shelled in the ocean a couple of miles long under those pressures could probably power every country on earth.
If 5000lbs generates three million volts, 15,000 pounds will generate Maybe, closer to 3 trillion. Per second...the only problem is transporting it. And collapsing the government and corporations of course...
With this topic low level power, it makes me wonder about power amplifiers. How can we amplify solar power - a magnifying glass? How can we amplify wind power? Is there a way to capture wind energy without more wind farms, and can we maximize the power generated? Makes me think of cilia of cells. Imagine if exterior building materials had micro surfaces for converting/capturing wind into power. R&D projects...
Well, if you haven't already looked, do a search for solar welders...yes it does work, yes it would be fun but damn, you get a gust of wind the wrong way and your arm would be laser off or worse.
But as we are on topic of the sea...
Another is the little known overunity effect of water itself. But as you're speaking about amplification...
The water hammer is the effect of resonant energy build up through an immediate stop to the flow of water. This does violate the supposed "laws" of physics. So again using just water, setting up a series of water hammers (even in addition to the cassimir effect chamber as above) the mechanical pressure can be used to generate electricity too. The problem with this method and the sea is that it would have moving parts, and erosion from the inside of the pipes would sand them down in very short order. Graphene might be able to withstand it, or perhaps fullerite which is or can reach 18 times harder than diamond. It just takes an awful lot of processing power to get it that hard.
But as hinted at, overunity has never been a problem, theres so many ways it is stupid. The problem has always been the people standing in the way of it being used. Once they are gone the so called energy race will be over in a single generation.
Piezo can be used, because similarily to casimir, it's effects turn log the smaller you get, so a suspension of high pressure nano quartz cut crystals subjected to mechanical pressures, again like from frequencies could easily generate trillions of volts. You might not even need the quartz if you just figured out the right frequencies to use for normal water. The sonoluminescence potential generated from the collapse of just one molecule of water gets hotter than the sun...I just don't know if anyone has figured out how to conduct that and repeat it a million times a second yet.