And got a degree in marine biology.
Expanding upon what I wrote before, that really applied to organisms at higher trophic levels. The lower down on the food-chain/web you feed, the more efficiently you use energy available in the system (lower trophic levels contain greater biomass). So if we're talking about, say, the equivalent of a
grazer/herbivore in terrestrial systems (most large whales also feed at a lower trophic level - they're usually secondary consumers, at about the third level), there may be something there. My money would be on large colonial organisms (like sponges, corals, bryozoans, etc.) with low metabolic rates and high levels of metabolic efficiency, which would allow them to grow to considerable size. But then again, I'm not sure this fits with your definition of a "huge sea creature."
Also, again, this applies to organisms living
exclusively in these deep-sea environments. If we have an organism that makes periodic migrations to more shallow habitat, that would change many of these considerations with respect to access to food/energy. But it would also probably increase the likelihood of its discovery up until this point (then again, this assumption may be flawed).