So, like in the original game, you need to grab a Chocobo to pass the Zolom and reach the Mythril mines. The swamp impedes your movement and Chocobos are supposably unaffected by the water so you need one as before in the original game. This is important because the zolom, a literal giant building-sized snake, lives in the swamp and can move at rapid speed. If you were caught in the swamp without a Chocobo, you'd supposedly have no chance of escape even if you weren't encumbered by the water. So, your only hope at this stage is to outspeed the snake on a Chocobo.
You get your Chocobo through an unnecessarily tedious mini-game and make your way back to the swamp. You reach the water only to find out that despite what has been said, the Chocobos are in fact affected by the water and move much slower than your normal movement speed. Technically faster than you would be without one but not by much. If this is fast enough to outpace the Zolom, the snake must actually be really fucking slow, which is not the case.
Regardless, you make your way through the swamp, with multiple Chocobos and a literal talking dog with cannine senses who should be able to sense the snake from a distance. However, the party walks onto seemingly dry land only to find that they're standing on the back of the building-sized snake without noticing. The argument could be made that the snake couldn't be sensed because it's under the water but that's not really valid considering they mistook the snake for dry land so parts of it were definitely not underwater and again, it's the size of a building.
The snake moves underneath them and somehow, at the exact same moment, everyone in the party manages to fall off of their Chocobos and the Chocobos now run off at full speed, now completely unaffected by the water.
This leaves the party to fight the snake in waist-high water. They should be powerless and unable to move because of the water, however, when the fight starts, the water is no longer an issue. The party can now move around completely unimpeded, literally walking on top of the water.
The snake is made out to be really intimidating, however, the fight plays out just as trivially as any other boss fight the game has offered so far. However, when the party gets the snake's health down to 0 a cutscene plays out, portraying how much of a struggle the party was having fighting the snake which was not how it felt in battle. They could have definitely changed the fight to make it seem more like they were struggling (as they should have been) as they have in the past with other bosses, but they didn't so this was really jarring.
The cutscene then shows the snake grabbing Cloud with its tail and diving down into the water. The water, I remind you, that was ankle-high moments prior in the battle, and waist-high before the the fight started. Now, however, the water is so deep that the literal giant building-sized snake is able to effortlessly dive down there with Cloud completely out of the party's view. The ankle-high water is now more like an ocean at this point.
Cloud wakes on the surface, the water changing again now to be ankle-high once more and the ocean he was previously in is now no longer to be seen. Cloud then claims that Sephiroth was here, and the party tells Cloud that he wasn't.
They then start worrying that something is wrong with Cloud's head, despite them witnessing the snake literally blast off into the sky, something Cloud couldn't possibly do.
The only person they know of who could possibly do something like that would be Sephiroth but rather than coming to the conclusion that he was there and they just didn't see him, they don't even bother questioning what happened to the snake and just think there's something wrong with Cloud's head which is really fucking dumb.
