When Susan goes to her room to recover her notes, she closes the door. Then she goes over to her cabinet, grabs her stuff, and gets nudged by a shark model. _Then_ the real shark comes after her.
First of all, sharks have to remain in constant motion to keep water flowing over their gills (unlike other types of fish). While it's a fairly large room, these are _big_ sharks. We get a good clear size-establishing shot of the sharks when Carter brings the one into the lab, and various shots establish that they are all the same size. Also look at how much water and stuff the shark earlier in the movie splashes up when it's in the pool. But the one in Susan's chamber moves around and doesn't displace any water at all.
Second, the water is only about waist-deep on Susan. Again, look at the shark in the pool filled with waist-high water earlier in the movie. The shark shouldn't even be able to _remain_ underwater. If she looked around the room, she (and us) should see it. What was it doing - hiding in a closet?
Third, Susan closes the door behind her when she enters the room. So if the shark wasn't hiding in the room, how'd it get in without her noticing?
And fourth...why does the shark wait so long to attack Susan? She's in the room for several minutes before it tries to attack her.
This scene just doesn't make any sense!