Shouldn't be any problem at all, assuming the bulkhead is strong. Icom's advice to NEVER mount the main unit overhead is intended to keep the crew safe, not just the radio
. It weighs over 10 lbs, and their point is that with the boat possibly slamming off waves the "weight" of the main unit increases by several times, and if it were to tear loose it could do some serious damage.
Just fine a dry place, preferably cool, with a strong flat surface to mount the main unit, and you'll be fine.