We were just at WDW in March (20-25) and bus rides from MK to the Dolphin NEVER took more than 10 minutes... usually less. We did this , often more than once a day, with our 2 year old... at all different times. Like I said, NEVER more than 10 minutes.
The MK area ferries do not connect with the Epcot area ferries at all. We did take the monorail from MK to Epcot a few times and enjot the walk back to the Dolphin from there!
The Dolphin was, in my opinion, a fabulous location. Walk or ferry to Epcot and MGM and easy (10 min max) bus rides to MK and AK as well as Blizzard beach.
Hope that helps...
Sara