Certainly is on the West coast, between the highwater and lowwater (mean). Pretty sure it is throughout Scotland, providing you do not wander into areas such as SSSI's or the like. A fair amount of my wildfowling is actually done off boats over deeper water or on the Machair when culling.
Taking firearms on ferries you are supposed to inform the purser (MCA requirement apparently). I certainly do not know of anyone who ever does, either carrying them on as foot passengers or in vehicles. I have been asked once if there was a firearm in the box and simply said it was expensive tools. Not strictly a porky as my firearms are tools of the trade when on control duties.
Not sure if a section 1 would be kosher wildlife and countryside act I believe unless its pest control and then only woodpigeons, Canada Geese despite being on the all year list are still protected from section 1 users sadly oh and from solid shot too!