Even before James Cameron refashioned himself into a real-life Steve Zissou, one always suspected that fictional storytelling was little more than a necessary nuisance for the “Titanic” director, a tithing he paid to exercise his real passion: Technical innovation. Remember, Cameron all but pioneered digital movie effects on “The Abyss” and “The Terminator” and once pawned his reputation to build a costly submersible film set on a beach in Mexico.

As such, the filmmaker is quite at home in the wondrous and thoroughly diverting large-format documentary “Aliens of the Deep.” Commanding a fleet of nimble underwater vehicles — each bristling with state-of-the-art 3-D cameras — Cameron and co-director Steven Quale plunge thousands of feet into the ocean to grab rare images of creatures that exist at sub-photosynthetic depth: Literally, where the sun don't shine.

Like kids at the zoo, Cameron and his guests marvel at milky squid, exotic jellyfish known as “space bagels” and other outlandish animals nourished (in lieu of sunlight) by thermal energy and minerals ejected from the ocean floor.

In one remarkable sequence, the team explores a cluster of hydrothermic vents (sea chimneys to the layman) buzzing with millions of hungry shrimp feasting on the bacteria produced there. Visually, “Aliens of the Deep” even outstrips Cameron's IMAX-produced “Ghosts of the Abyss” — this isn't a cold, broken wreck we're exploring, but the mysteries of evolution itself.

Cameron comes off as enthusiastic and affable, though not particularly well-informed (“I don't know what that is, but it's incredible!” he says, or something to that effect). The same could be said for his so-called “expert” crew of marine biologists, seismologists and astrobiologists, selected more for their youth and looks, it seems, than their expertise.

Consequently, the audience is treated to such glimmering scientific insights as “Aw, yeah!” and “This is the bomb!” when all we really want to know is the name of the exceedingly ugly fish staring us in the face.

Still, Cameron's main thesis — that deep-sea exploration might provide insights into extraterrestrial exploration — is compelling, capped by a thrilling sequence imagining how life might look buried under miles of ice on the Jovian moon Europa. The sequence, of course, is computer-generated — Cameron will have to come up with a few more technical innovations before he makes that journey.