The High Priestess, Part 1 Episode 23

Jotaro and the others obtain a submarine as a new means of travel. Through the Red Sea, they try to head toward Egypt where DIO awaits. Using the sonar to detect any threats on the outside, the submarine quietly dives in. They however realize that if they're attacked underwater, there is nowhere to escape, so they are on their guard. Finally, when they are about to surface, a coffee mug that was in the cabin transforms and cuts off Joseph's artificial hand.