The boat will take you close to an hour. If you hop on the boat at IG, then it goes to BW, BC , and then Swan/Dolphin. Then it goes to MGM.
You may be able to walk it,briskly, in about 30 minutes.
If everything goes perfectly,you get on the boat just as it is leaving IG, not many people on it getting on and off, you may be able to do it in 35-40 minutes. I have always found it to be a looooooooooong trip.
That said, it is still faster than any other way on Disney transport.
Leave plenty of time. When you get into MGM, it will take you another long walk to get to the show. So lets add this walking up. From Japan to IG,a good 5-10 minutes, take the boat 35-60 minutes , the boat to Fantasmic, at least 10 minutes.
If you could do Fantasmic the same day you go to MGM and do a dinner there you would get the dinner seating in Fantasmic. A much more leasurely (? sp) pace.
Good luck.