Enter Park.
Go to boat ramp at Apgar Village. Enter from dock of boat ramp and swim about 100 yards to the east at 40 feet or so and you will find the Debrie field.
Or, go behind the Lake McDonald Lodge, enter from the dock and go where you will. Or, go to the Sprague Creek camp grounds do a beach entry and swim north along the shoreline about 20 yards off shore in 60 feet of water and you will swim into the underwater forrest. Tree trunks from the bottom almost to the surface.
Facts about Lake McDonald in Glacier National Park