Yes that is possible and I would say it could take a total of 30 minutes or maybe more because of these reasons. Waiting for the bus at CSR, then waiting for the boat to arrive at the Grand Floridian. Don't forget you can also take the Resort Monorail to the Grand Floridian which will come a lot faster then the boat incase you just missed it. Although you will be stopping at the Contemporary, TTC, and Polynesian before arriving at the Grand Floridian, where as the boat goes right to the Grand Floridian after it leaves the Magic Kingdom.
Of course another option you have is taking a taxi from Coronado Springs to the Grand Floridian, which should take about 15 minutes and you would also be willing to spend the money for a taxi.