I'm not sure about a shuttle, but I do know that Mears makes runs from Disney hotels to Sea World - we used them last year. You can make ressies through them for an early morning pickup and then name your time to come back. If I remember right, I think it was only $10 per person.
One point to note: we have consistently found Mears to be very unreliable in their pick up times, despite the fact that we make and confirm ressies. If I knew of other more reliable options, I'd certainly give them a try, so I'll be checking back on this post to see if anyone does know of other available shuttles.