I've found a way to get to the upper parts of the aquatos sewers without the gravity boots. You need the hypershot and have the helipack equipped. You know those bridges with the yellow platforms that overlook the green water that you swim down? Go to the one that has a hyper shot DIRECTLY above the top platform(it's the second to last one).
Now here's how you get to the top bridge that has the hyper shot target.
Once you are on the bottom bridge, you should see two things near both ends of both entrances to the bridge: a brown railing, and a green pipe. Jump on the brown railing, then jump on the green pipe. I may take a couple tries.
Once you are on the green pipe, Boost jump up to the second level. Jump on the green pipe on that second level and equip the hyper shot. Enter 1st person mode and target the hyper shot target, then swing across to the other bridge. You should now be able to access the upper aquatos sewers.
Hope that wasn't too difficult to understand... >_>