High pressure electrolysis (from wikipedia)
High pressure electrolysis is the electrolysis of water with a compressed hydrogen output around 120-200 Bar (1740-2900 psi). By pressurising the hydrogen in the electrolyser the need for an external hydrogen compressor is eliminated, the average energy consumption for internal compression is around 3%.
Toyota fuel-cell car in 2014 with 300 mile range and Tesla-competitive price. (The pressure for this tank is 690 atmospheres). One atmosphere equates to about 10 meters of depth in the ocean so theorectically you could get this from electrolysis in the deepest ocean trenches but this isn't likely to be practical. OTEC requires accessing water at about 4C, which is also its greatest density, to be efficient and this starts at about 1000 meters virtually universally in the ocean. One hundred atmospheres is likely the best you can expect, which isn't bad because the best OTEC sites require the production of an energy carrier to bring the power to market in any case.