The "per-linear-meter power density" what some would call "wave energy flux" represents an order of 5x greater energy density than wind, which is roughly 10x more dense than solar. When T Murphy describes "third string solar" he highlights that a quantity of energy is lost in each conversion, yes but also the density is increased. Similar to following energy starting with biomass, fermenting to dilute alcohol, and distilling to pure ethanol. In this case there is no effort or external energy required for incident solar energy to generate a lesser amount of denser wind energy, and for wind energy to create a smaller still amount of high density wave energy. However, this quantity is still large enough that even a fraction of it converted to electricity represents an quantity of power that I strongly reject to being called "nil" or "puny".

Following a thorough analysis and R&D phase, the important metric, levelized cost of energy, relates to the quantity of material required to construct a device that interacts with a given quantity of power. Operating in a more power dense medium favors lower LCOE. There are challenges with the salt-water environment, that fact doesn't preclude the existence of industries such as trans-oceanic shipping, offshore oil and gas, navigational and observational buoys, and other such endeavors.

Personally I consider it a good thing for an energy technology to be distributed throughout the world, I think it is preferential than having the entire energy resource concentrated in one part of the world. Another benefit it offers is that its availability is decoupled from wind and solar, the waves don't stop at night, and once established continue traveling without wind.

There is no silver bullet and wave energy is no exception, there will always be a finite quantity recoverable, and a certain cost to recover it. However, to determine those specific numbers would require a herculean effort to thoroughly analyze all of the possible wave energy converter designs - which consist of a number of major of typologies, and within each topology an even greater number of specific designs and sizes, each with their own cost and performance, which also varies depending on seastate - you would need to analyze every possible device not just for power converting performance, but for an estimate of suitable materials and construction techniques, and their costs. Only then could you answer the important question, can any quantity of wave energy be economically recovered at a cost competitive with other leading renewable energy sources.