The location of your tuner makes very little difference Radio-wise.
I am assuming that you have the grounding to water
done properly and a long whip antenna
or isolated stay.
If you move the tuner a bit (like half a meter or so) it will not change anything in terms of radio
reach, but as your friend points out it could make lifetime of the tuner longer.
Another good idea is to check the tuner casing and make sure its really watertight, even tiny amounts of saltwater can cause failure, I happen to know from experience :-( (Mine was in the transom compartment, above WL but still exposed to salt