I don't have a boat like yours , but I have mounted a dome and used a wedge to make sure it mounted level while on plane . I would go out on a limb and think that if mounted in middle of roof the signal may clip the roof and have a blind spot right in front of your craft, were mounting up front it would not, but then behind would have a blind area, which I can see from lots of mounting this must be ok. Personally have mine on a pedestal.
Hope that helps, enjoy the new toy.