Basically without a car you have three options:
1. Cab - easy to catch one at Universal for the return
2. Mears Shuttle - previous posters have mentioned this, it's about $20-30 return.
3. Public bus. You can catch the 50 bus from DTD or the TTC and then get off at Sea World. From there, you can catch the 8 bus to the corner of Universal Boulevard - opposite Wet n Wild. From there it's a 10 minute walk. That whole trip will take you around 1 hour, but it will only cost you $2 (ask for a transfer when you get on at Disney, it's free).
If I were you, I'd do #3 going and #1 returning.
Actually, if I were you, I'd book a night in a hotel in or near Universal Studios - it's an awesome place and well worth a night or two up there.