Mary is one of those films that sets sail but has no real sense of direction. Its biggest short coming in my opinion is the premise is sound but the setting is wrong.
A modestly sized family yacht, simply doesn't provide enough physical space and as a consequence scope, to meet the expectations, this horror aspires to. Its as if, for example, Friday the 13th, was set in a phone booth. its far too constrained and has to fall back on drawn out physiological tension, in a vain attempt, to compensate.
The result is a slow, predictable and ultimately, rather anti climatic film, that takes the viewer nowhere interesting. That's in spite of a strong cast, quality acting and decent cinematography.
Marginally watchable but that said, there are far better horror films with a nautical bent on offer, like 80's classic, The Fog. 5/10 from me.