It is all a question of what 'duty status' the young man was under. Being on an exped does not mean you are 'on duty' at all times, particularly when chilling at the beach in this case. I am involved with planning major overseas tours for a RAF sports team and you have to specify exactly when and when you are not 'on duty'. For example, you would be only regarded as 'On Duty' (for the MOD Compensation scheme) when you are actually taking part in the sport you are touring for. Any evening time, meals out, stand down days, etc you are unlikely to be covered.