I agree that the bus to MK, then monorail to GF is probably the best bet. However, I would
budget at least 1.5 hours for the trip. According to the UG, the trip from the Swan to MK averages 38 minutes, with a worst case of 50 minutes. The trip on the monorail should take about 18-23 minutes with a 10 minute walk to the monorail station. So best case is about 66 minutes and worst case is about 83 minutes!
Just a note, the UG does not show times for MK to GF, so I used the estimates of Cont to MK instead.
My limited expereince with the boat from MK to GF is that it was slower; usually because the wait for the boat was much longer than the wait for the monorail.
Best of luck -- Suzanne