In the movie, when the Poseidon capsizes, the hole that was blown in her would have only caused the ship to sink as the Titanic did, only by the stern, not the bow.

This would be wrong, as stated by James Martin the ship had taken on extra ballest because of the storm they were in. If the watertight doors worked as they should have, one side of the ship would have gotten heavier than the other. Water still flooding in on one side of the ship plus extra ballest equals ship capsizing.

The only reason the ship would have sunk the same as the titanic did is if both bombs went off so the ship would be evened out with weight on the Port and Starboard side.