From a historical aspec, it would not be dangerous. But actually doing this, would be dangerous. Plus if you all recall, the USS Holand ran on gas. Very dangerous. Which is why they switched to diesel.
I have a memory of reading that after WWII some Italian midget sub people kept theirs and used them for smuggling cigarettes.
FTM doesn''t North Korea have a home brew submarine class used to smuggle infiltrators into South Korea?
Previously I mentioned that Ellsberg novel. I could see somebody towing a submersible below a cargo ship. One or two crew could monitor buoyancy, it might have power for it's motors from the
ship's gnerators.
I recall that The British had transit crews for their XCraft which were sometimes towed to Norway for the attempts against the
Tirpitz (?). The transit crew was exchanged with an operational crew close to the target.
This is possibly a dangerous thread to pursue.
Larry Murray