Ideally, what you want is an RF counterpoise isolated from the boat's DC ground -it could be argued that it's all interconnected regardless but not necessarily if your thruhulls are not bonded to the engine
. In other words, the RF counterpoise and the boat's DC ground should be separate. Many don't bother and it works fine for them.
Potential issues with a common ground/counterpoise are stray RF getting into electronics
(more likely than usual), induced electrolysis
on the radio
Consequently, in the long run, you may be better served to connect the radio ground to the DC ground bus and the tuner to the thruhull. This may seem counter-intuitive as they are all electrically connected someplace but there are sound reasons for
physically separating them.