This is a massive plot. They knew Gargantua would slow down time on that planet, but they didn't know about the massive wave it would create when they saw it was filled with liquid (water?)?
it wouldnt affect tides, not waves.... its possible that it could be a giant tsunami wave that would circle the "world" with no land to absorb energy it would move really fast. or maybe high windes prior and and the low part is just pushed ahead of the big wave... idk, its a movie
The tidal wave is locked on the gravitational pull of Gargantua as the planet rotates so it looks like it's moving when in fact it's 'still' and that's the planet rotating.
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