Outside of the semi-submersible 'narco subs', there have been few cargo-oriented submarines. The main ones were made by Germany during WWI or Japan during WWII (notably the Maru Yu class [1]) and all had a conventional format with diesel/electric propulsion. I think that underwater gliding, which uses wings and variable buoyancy to glide up and down in a saw-tooth pattern, could greatly reduce the energy needed for cruising.

I thought about enlarging an underwater glider like the Sea Wing [2] but the repeated pitching motion might damage the cargo or make it shift in the hull, with catastrophic results. A possible solution is to use variable-incidence wings in order to keep the cargo hull horizontal, but this requires the outer hull to be streamlined in both the horizontal and vertical directions, like an oarfish.

There is at least one project of unmanned submarine aiming to transport valuable cargo in the Irish Sea [3] but it seems to be a collection of startup-style grant-seeking buzzwords. My proposal would be much larger, with a pressure hull diameter comparable to a jet airliner in order to accommodate unit load devices and shipping containers, and designed to allow front-loading from the upper hull.

EDIT : updated the 3-view plans with a catamaran configuration, which is more stable on the surface and gives twice the useful volume for the same width as the outrigger configuration.

[1] http://www.hisutton.com/WW2-Maru-Yu.....Submarine.html
[2] http://www.hisutton.com/Chinese-Sea.....ine-Drone.html
[3] https://www.bbc.com/news/uk-norther.....eland-58571143