I’m looking for a book that I believe was aimed at middle-grade readers. It involved a group of siblings who discover a sunken playhouse and the ghost of a boy who died by drowning…he allows them to breathe underwater somehow so they can play with him. What really sticks out to me is that the lake the adventure takes place in has a huge snapping turtle that poses a significant danger to them.
Lost Classics of Teen Lit: Name That Book! 
Not all of the details match up, but this sounds a bit like Magic By the Lake by Edward Eager.
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