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Been stung many times, fortunately not by a lethal jelly. The issue with the lions mane is that their tentacles, the part with there stingers, can be so long that you can't see the jellyfish and still get stung.
Our ocean is rough and 40* f so not the ideal conditions. Used to travel 200+ miles to protected waters for most classes. Once a person gets good then we can take them out in the pacific.