A maar forms when magma interacts explosively with water, excavating a crater below the surrounding ground. Such depressions may hold lakes, as at the Killeak Maars in Alaska. Water is therefore a possible occupant, not the definition of a maar.
Compare the low rim and broad hollow with a cone that builds upward. A lake alone cannot identify volcanic origin: the surrounding deposits and geology supply the evidence. This entry shows that an eruption can remove terrain as well as add it.
Source / photo: NPS · Public domain. Resized. No claim to original U.S. Government works.