Yes, you can walk from the boardwalk to MGM. If you are staying at the BWI, head over to the LUNA pool and walk out to the canal. Turn left and follow the canal to MGM. It's about a 10-20 minute walk depending on how fast a walker you are.
Just wanted to say how much we enjoyed the walk to/from MGM when we stayed at BWI. It was very convenient not having to wait on a boat, plus the walk itself was a pleasant experience. We didn't even mind it after a full day of touring MGM!
as long as you do not mind walking i have found the walk is actually faster than the boat. i had a dinner reservation at the boardwalk and i was in mgm and noticed the boat would be too full so i did the walk and i beat the boat by a few minutes.