Soooooo......

[Ghoulish Delight]

I think it's a combination of an unfamiliar unit of measure and the choice of phrasing. In general when someone says, "X units of distance under the seas", people's first thought is depth. That title probably the ONLY instance I've ever heard of using "under the sea" to mean "lateral distance traveled while under the sea."

If it were "70,000 Miles Under the Sea" I still think there'd be confusion. It would be as persistent but it would be of the form, "Huh? That doesn't make sense the sea isn't that deep...oh, wait, does he mean 70,000 Miles WHILE under the Sea?"