Brad,
The general idea is to make the 'angle' as shallow as possible. That will depend on a lot of things, so the drive shaft being parallel to the water surface isn't always possible. 'Close' does count, though. The only 'angles' that really don't work well are those close to 90 degrees with the surface of the water, and those that 'point' up to the surface. Anything in between will work, sort of...
- 'Doc

[It's early! Not enough coffee yet. Sorry 'bout that last part.]