That sounds like an awesome thing you want to build there.. had something like that also in my mind but couldnt really think of something to avoid this cable length...
But i had a few ideas though...
Maybe, and i have not asked anyone if it is possible, you could amplify your USB -> Cat5 connection by using some op amp like for example the LM358.. but i guess in the end that would be much trial and error (but as those op amps cost only a few cent it could be worth it..)
Then my second idea was using wireless stuff.
OceanReef has a few examples on under water communication systems which base on low Mhz transceivers.. that would have been my second idea, but if that is at all or in reasonable price ranges doable.. that is a completely different question..