This may be a situation where you would consider Gimbal lock – Gimbal lock - Wikipedia
As for the relativity of the HPR values, I imagine it would depend on how your boat model is parented in the scenegraph. Absolute and relative positioning are available, I believe, depending on how your graph is constructed.
Obbo’s Descent Obbo's Descent (pyweek 30 submission) would seem to have a solution to this issue as a core game feature, so perhaps one of its developers could explicate the details of how that functions.