If you thought your yacht was too small for a submersible, the new U-Boat Worx Super Yacht Sub 3 will deep-six that sentiment. The same Netherlands-based builders that introduced the C-Explorer 3 last year have billed their latest addition as the most compact three-person submersible on the market—besting their smallest previous offering, the HP Sport Sub 2. The approximately 10-foot × 8-foot craft weighs less than 8,300 pounds and fits in a standard shipping container.

Another feature that competitors have…