Last time we went there were 2 taxis waiting outside, dont know if they had been booked or they just hang around waiting for a fare.
I would thing Discovery cove staff would ring for you one there are always plenty of them around when leaving. Or if you know what time you are going to leave prebook with a taxi company.
I don't think you'll have a problem. We didn't book in advance last November. When we came out there were taxis waiting and the staff were very helpful.
We pre booked a taxi both ways on our DC trip. But saying that we had a definate return time to WDW as we had ADR's for California Grill as it was our 5th wedding anniversary.