Difficult to say. We were there to catch motorbikes and chase them off the sand dunes which would be pretty stressful for him anyway. He wouldn't go anywhere near the sea at first but did end up with a gallop back in about 8" of surf and seemed pretty settled afterwards. He's not 'bombproof' at all but is a quick learner and once he's decided something isn't going to hurt him he tends to accept it and move on.
I'll find out next time I take him, if he wont get out of the horsebox I'll know!!