I'm not sure that's a viable day trip, quite frankly. There are a couple of options, and please remember, reasonable is relative.

1. Charter flights between Anegada/St. Thomas. My husband and I did this on our wedding/honeymoon trip as a special splurge day trip. We stayed on St. John, so we had ferry & cab rides on both ends of that day trip. We've also chartered flights between St. Thomas/Anegada when we stayed on Anegada for the entire vacation.

2. I think there used to be a speed boat that was available for charter.

Another option would be to consider spending more than a day on Anegada. Perhaps think about staying for a couple of nights so you may be able to catch the ferries between Red Hook/Road Town (BVI) and then Road Town/Anegada. The downside with that is the reliability of BVI ferries (we have experienced that personally and witnessed others who were stranded).

Feel free to PM me for more info. We've chartered flights between STT & Anegada, and we've done the ferries. Personally, the charter flights are the way to go, especially if you get a group together to bring the per person cost down. I believe Bill Hartzman on the BVI TTOL Board is a good resource for charter flights out of San Juan.