Thanks for all the advice here, I think the guy from icom was worried about sea water corroding the seals behind the knobs, however as my rib is garaged and we tend to go out in the better weather I think the chances of it getting continuously swamped will be quite slim.
Hereís a few points from the icom leaflet:
Water proof to 1m for 30 mins
IPX7 equivalent waterproof construction
AquaQuake draining function (which sheds water from the speaker)
I have read the manual and I canít find anything in there about where it should be mounted.
At the boat show I looked a £220k Scorpion cabin rib and that had an almost vertical icom M411 installed on the console.
Itís a shame icom donít make covers for their radios.