[bobthepanda]

I figure once you join them, you could also apply waterproofing to the outside as well, no?

[lmm]

Maybe. What would that look like, adding more gaskets on the outside? That sounds even harder to maintain since the only way to get access is diving.

[tokai]

The sections are laid in a trench and covered by sediment. There's not going to be any routine maintenance by divers.

[euroderf]

Some kind of goop that would get very slowly squeezed into gaps by water pressure ?

[usrusr]

Sealing is really not that difficult if you have access to the high pressure side. The hard part is identifying the location of the leak. In sum, this means that they have to absolutely nail it, on the first attempt, for the bottom part that is resting on the sea floor. If they can to that, the rest of the circumference will also be so good they don't have to even think about fixability.