I imagine that you can fly solo at Biscarrosse because the operator there very closely controls the flying. In particular, because there are only two possible lakes to land in. They can give you a good briefing about landing in each lake as appropriate, and they can be confident about the conditions. Once they decide that conditions are suitable for solo flying, that would apply to all the places you could land.
I flew an amphibian across Europe with a friend a few years ago, and there are surprisingly few places where water landings are permitted. I think the Germany has four places. We headed to Lake Como in Italy, but they have very restrictive hours of operation. Had we landed, we would have had to stay for a day or two until the float base opened again. I did land in the harbour at Split, Croatia, though that required a lot of permission requesting!
The best opportunity for diverse float flying will be found in the Vancouver/Victoria area of British Coumbia or in Ontario. Anchorage is another good place to enjoy float flying.
Things are very different in other parts of the world, where you can take off one lake, and land in any of a hundred lakes within an hour or so flying. The lakes can vary greatly in size, condition, and hazards. I can take off from my home runway, and land in either of sic different lakes within a five minute flight. They are all different, and it could be safe to land in one or two, and totally unsafe to land in the others. One another day, I could safely land in any of them. Or, you fly in to your favourite lake in the morning, and by the afternoon, it's not safe to takeoff. More than once, I've been relaxing on shore, and decided to leave with no delay because of worsening conditions.
The local float flying school will let you fly solo in their lake, but no other lake, unless you've been checked out by their instructor in that lake. The practical reality is that although you might be checked out in another lake (maybe where your cottage is), you cannot practically rent the plane and "fly to your cottage for the weekend", because the school has to have the plane in service the whole weekend. So renting a floatplane to go somewhere is wasteful unless you can land there with purpose. You can use a landplane for a touring trip. If you're paying the extra to take the floats with you, you may as well make the most of it, and fly circuits!