Given that propellers are moved by shafts connected to motors within the body, wouldn’t the interface of these shafts with the shell of the vehicle be a weak point? How do they waterproof and pressure proof those joints?
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Yes, it is a weak point. There are seals in place around the shaft that minimize the leakage. There are pumps on the inside of the sub that pump the water back out to the sea.
In the case of deep submersibles, there is no shaft passing through the pressure hull. They use external thrusters with electric motors.
Submarines and submersibles are different categories of underwater vehicle.
In submarines, propulsion machinery is inside the pressure vessel. Submarine propellor shafts are sealed with stronger versions of the same kinds of shaft seals that ships use.
With submersibles, propulsion machinery and batteries are outside the passenger pressure vessel. Other than hatches and portholes, only electrical connections and (on some kinds) a manual ballast release permeate the pressure hull.
Pfft! They put extra coats of wax on them. Also helps them from being detected when they’re super shiny.
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