More comprehension problems.
Given that they'll be at near the lowest point in the door frame, I'd say their sealing would be of prime importance.
The thing is that the bottom part of the door frame will be easily sealed, being all steel, and will have large drain holes. Now the only problems that could arise are if the water level can rise above this part, which really shouldn't happen. So trying to seal the entire door so it could be filled with water seems to me unlikely as well as unnecessary.
IMHO, the reason to seal the flexible liner is more one of draughts and possibly noise rather than to prevent water leaks.