Still a no-no. You can't waive the reqts of Rule 5 even with 'permission' - save CAA display authorisation or equivalent.
IMHO to fly lower than 500' over a large expanse of water, in a serviceable single engined light aircraft - you'd have to be INSANE, stupid or both. There are just too many 'what if?'s for my liking.
You want to fly LL? Join the RAF. It is fun, and I can understand the temptation. However, exercise some self-discipline here and you will definitely live to do the fun stuff!
Before y'all write me off as a complete fuddy-duddy, try something which has always helped me when I am in doubt as to the sense of a particular course of action:
Write the accident report in your head, and if your reaction on reading it would be either "What a tw@t", "Serves him right" or something similiar (we've all thought it at one time when reading other BOI reports!") - then DON'T GO!
Hope you manage to continue to have fun in your flying regardless.
AB