If...you are at the BC dock as a boat is loading, then it may be faster to take the boat. Same thing for the return to BC from MGM. But, if you see a boat pulling away from the the docks as you walk up, then you'll have another 20 min wait before the next one comes along.
I have made the walk (and I use that term very loosely in this case!!) from the Intl Gateway, to MGM front gate in 17 mins and that included a stop in my BWV room!!! But, it wasn't a walk....more like a race walk. At a normal pace you should be able to make it in 25 mins!!! It's a nice walk unless it is very humid/hot, raining or you are really pooped from a day in the parks!!! If these are the case, then take the boat.