I was talking about this the other day, as I had one last week.
I don't go when I've got one. You're not supposed to get it wet. Even in the shower if possible!
It's not good for the blister on your lip, but also, as it's highly contagious, it's not good for other people potentially!
Someone did recommend the patches - Compeed? No...I think that's for blisters! I can't remember the name. I will see if I can find the thread for you. I think they provide protection against water, but I didn't try them in the end as I didn't want to risk getting the sore wet.
them patches are good, and with them on you are fine to go swimming, i think hellie was right i am sure they are called compeed, and the good thing with them is they make the coldsore look less swollen, and you can put make up over them too.