I'd never taken a glass-bottom boat. To be honest, the idea intimidated me at the beginning. What if the glass couldn't support the passenger's weight? What if a shark swooped [...] and broke the glass?
I searched on Google and found that swoop in is the most common phrase. But I don't know, maybe I should write swoop up since the glass is at the bottom of the boat?
You can use whatever adverb of direction (or particle in a phrasal verb) with swoop that makes sense in the context, just as you can with any other verb of motion. So all these make sense, just as a few examples:
For actual prepositional use, you’d need something like swooped up the maelstrom.
Use "up" in this case (swooped up, swoop up), because the shark is coming from below, ostensibly.
However, I would not use "swoop" to describe what a shark does. Swooping typically describes the action a bird or another animal would make where it flies down without using any flapping motions or motions to make it continue its downward arc - you can use this to describe other animals, or people as well, to give a similar idea of what happens (She swooped in to grab her child from the maw of an Alligator)
Swoop as defined by dictionary.reference.com (http://dictionary.reference.com/browse/swoop) specifically also states that it's more of a diving motion than rising from below, which is what a shark would do.
