For a JAA PPL there are no requirements specifically for an over water flight, other than the normal weather conditions.
You must file a flight plan for an international trip (and in Ireland-not sure about uk-must file a flight plan for a trip over more than 30nm of water).
I understand an National PPL is different, and an international trip is not permitted (possibly some international mutual recognition-I'm not too familiar with it, but generally no international trips).
However various clubs will have their own requirements. Virtually all will require a "cross water checkout" before allowing you go beyond gliding distance of land. The contents of any such checkout is completely up to the school themselves.
I'm told mine will require an "instrument checkout" too, before allowing me to do a cross water checkout. Seems a bit like over kill to me....but that's another story altogether
dp