While not as close as Epcot, the walk from the BWV is very short and leisurely. From the elevators, you just exit through the Luna Park Pool doors on the lobby floor. Veer to your left and follow that path to the service road next to the canal. On the other side of the service road is a walkway that follows the canal to MGM. As soon as you reach the underpass, you'll see the main gate to the theme park.
Your other option is to catch the boat at the Swan/Dolphin dock. It is nearly as close as the Boardwalk dock and (unlike the BW dock) is the final stop for the boat before it travels over to MGM.