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.