Following this route: Polynesian - World Drive to Epcot - but DON'T go to Epcot - take the bypass to Downtown Disney Area (route 536) - Stay on this road (Epcot Center Drive) and leave WDW - across I-4 take a left on International Drive (watch for signs) - follow I-drive until you see signs for SeaWorld, it's on the left. - if you get a WDW property map you will see the route I have outlined here.
It should take you a good 20-30 minutes to get there - without heavy traffic. Using that route will avoid the I-4 crunch as that streatch of I-drive is not very built up yet (but it's getting there).
-Kim