Yes, there is actually a way to get there by bus. It is the Lynx City bus system.
First go to
www.golynx.com
At that website look up the route and times for Lynx #50.
You will see that it runs from the TTC to Downtown Orlando and one of the stops is SeaWorld.
So, first thing is take a boat from Ft. Wilderness to Magic Kingdom or the Contemporary, then get on the monorail and get off at TTC.
Once at the TTC ask the first cast member you see where the Lynx bus stops are and proceed that direction.
The ride is $2.00 per person each direction. (Children may be less, not sure). When you get on the bus, tell the driver you want to get off at Sea World.
(FYI, it is the third stop after you get off I-4). There are intermediate stops on the route that are not on the map.
Then walk across the SeaWorld parking lot and you are there.
When you are ready to return, walk back across the SeaWorld parking lot and cross the street and you will catch Lynx #50 back to Disney. There is a stop at DTD Westside. I got off at that stop and went to dinner at DTD after leaving SeaWorld. It also goes back to TTC if you just want to go back to the Fort.
Most of the rides and attractions can be done at a leisurely pace in one day. However, most of the time the tickets are "2nd day free", so I did go back another afternoon to catch one more show that I missed.