Can I fit a Skin Fitting with Tail below the waterline ?

NO
All Skin Fittings fitted below the waterline must be threaded and connected to a Ball Valve to enable shut off in event of an emergency and when unattended at the Marina. Additionally, 2 stainless steel hose clips should be used on every tail fitting and periodically checked for tightness as they may loosen off as the hoses age.