That's the most stable capsized ship EVER. It's upside down and you can walk right down the inverted hallway as if it was in a ground-based building! As a matter of fact the ship isn't just upside down - it's precisely perpendicular!
That's somewhat justified; the ship naturally wants to rest that way and flooding of the upper decks stabilized it. Unfortunately, that same flooding is slowing dragging the ship down around our protagonists. Worse, when Robert floods the ballast tanks, the ship does become unstable, and eventually turns back over.
If Robert didn't want Jennifer fooling around with Christian, why did he invite him along on a cruise with them?
If the rogue wave was as big and destructive as it was to one large ship out in deep water - what nearby place/ship was intact enough to send the boat and the helicopters?
Depends on where they were; ironically, for an earthquake-generated rogue wave, the farther out to sea the ships were, the more likely they wouldn't have even noticed the wave. Closer to shore, though, would have been a problem.
There is something weird with that kid. You would think he would have been lost in tears after the crap he goes through. He can hold his breath for a LONG time. Doesn't complain about all this running and swimming once. They finally get a break to rest and he wanders off.