I DNFd my 1st open-water tri & I clearly hadn't done enough OW sessions. It took me a while to get used to lake swimming as well as in a wetsuit (even though it fitted me). At the start of the following season, I went for an OW session with someone who knew what he was talking about & it all started to make sense.
I understand it's not always easy to get to a venue and you certainly don't need to be perfect on your 1st event but it's definitely worth doing at least once just to give you the reassurance that you probably won't get in a pool.
Most (if not all) races in France will ask for something but as long as there is a stamp from some sort of health place & a signature I don't think they check too much. I also read a while ago about Italy asking for something similar but haven't raced over there so don't know how fussy they will be.