The boat ride is around 8 to 10 minutes if my memory is working correctly. Obviously, the monorails run somewhat more often. The boats seem run every 15 to 20 minutes or more especially during the day. The monorrail holds more folks. They seem to run every 7 minutes or so. It does depend on what resort your are staying on how quick you will actually arrive once you board the monorail. 1st stop - CR, 2nd stop - T&TC, 3rd stop - Poly, and the 4th stop - GF. All of the stops come with the associated unloading and boarding waits. Also, occasionally, during the day you might get caught in one of the "safety checks" that can cost you some wait time. One advantage with the CR is that you can walk back in about 7 to 10 minutes without any wait.
We have done both and it was with small ones....we have 2, 5, and 7 year olds. The monorail is most likely a bit quicker but the boat isn't terrible.
Hope this helps.