Something about this? “Once deployed, the vehicle uses efficient, buoyancy-driven gliding to move through the water..."

Sounds like it gets around by changing buoyancy, which is pretty cool and also probably completely silent.

The real benefit is it works well with harvesting energy for propulsion from the depth based thermal gradient.

It's still much wider-bodied than any glider I've seen. I'm guessing this is because of the need for larger payload bays.

The answer is probably explosives, but are there other payloads you think this thing might carry?

Comms? SIG-INT equipment?

Given the autonomous nature I'd be surprised if it were used offensively anytime soon, so I'd guess the latter.

You don't need as much wing-area underwater as you need on the air.

Same reason fish, marine mammals, submarines etc. need fins.

To look like a manta ray?

The same reason that rays and skates have wings, they're useful in any fluid medium, even when they aren't generating lift.

> even when they aren't generating lift.

Yes. But these do generate lift. This is basically an underwater glider[1]. It can change it's buoyancy which would make it sink/raise straight down/up. It uses then its wings to turn this vertical speed into horizontal propulsion. It basically flies underwater in a sawtooth pattern (when seen in a side view).

This is a very energy efficient way to go the distance. These machines can loiter for months on a single charge typically. They only need to spend energy on the top and bottom points of their vertical swoops to change their buoyancy.

1: https://en.wikipedia.org/wiki/Underwater_glider