Rob, many of the ocean areas conducive to OTEC are also increasingly becoming dead-zones due to eutrophication and over-fishing. I see the oxygen in part being a solution to the eutrophication problem and if Boyce is right, and we are losing the initial source of the atmosphere's oxygen and a current source of 50%, then that which is not dissolved en route to the surface would replenish the phytoplankton shortfall.
Hydrogen is hard to transport but it can be converted at its source to NH3 or CH3OH which are not but are non or low carbon transportation fuels. My preference would be to work of the transportation problem however because hydrogen is as much a water currency as it is an energy currency and the production of both at the point of need addresses two needs.
I became interested in OTEC in the 80s while working on a book plotted around the extraction of the ocean's vast, dissolved, resources which OTEC could make available. The interest was revived a few years ago when thinking of ways to combat sea level rise and more recently by the potential to limit storm damage.
OTEC could address the sea level problem five different ways even as it provided all of the energy we need; converting heat to work to reduce thermal expansion, electrolysis to convert liquid volume to gas, desalination - OTEC Open Cycle - and terrestrial use of ocean volume, movement of surface heat to the depths where the coefficient of expansion of at 4C is half what it is at the surface, and the sapping of ocean surface heat that causes storms which in turn are the main driver of tropical heat towards the poles, which in turn is melting the icecaps and permafrost.
It is a costly approach, but again the heat pipe reduces the main driver of this - the cold water pipe - from a diameter of 14 meters to produce 100MW to 1 meter and thus the supporting infrastructure can also be massively reduced. And I believe its cost deserves to be offset by the savings derived from limiting SLR and storm damage.
With respect to stored thermal energy I trust the following two diagrams are self explanatory.
Re cost effectiveness, I fully agree with IK's most recent post that, "prototypes should be built and tested that's the only way to encounter the full range of problems you would not predict."