[dredmorbius]

Subs are boats.

[nevir]

They're subnautical vehicles. They're beneath that.

[johnhenry]

The German name, "Unterseeboat", and the English adaptation, "U-boat" both imply that they are a type of boat.

[dredmorbius]

I'm starting to suspect some dark humour here. But it runs silent, runs deep. And seems to have gotten all wet.