Things like this should be perfectly legal as risk is part of life anyway. We should be able to decide what risks we are willing to take and what risks we aren't. I'm sure driving is actually more dangerous than jumping into a lake for a person that knows how to swim, but driving is still legal, so why would enjoying nature not be?
Now if a particular lake or tree offer particularly high danger to the safety of people there, than some restrictions might be warranted, but that is not the general case. The only other reason not to enjoy nature would be if you are destroying it by doing so and I would also support restrictions if the flora and fauna in the lake is suffering significantly because of human leisure activities there. But besides that, it should all be fair game.