it depends on the quality / level of pollution in the lake you plan to take them from, maybe take a water sample and test it.
Also, even if the plant you get looks healthy, clean it thoroughly, take out anything rotting/decaying on it, and I'd recommend giving it a bleach cleaning. I've never bleach cleaned before so I'm not sure how that works, google it lol.
Otherwise i'm pretty sure it would be safe. It would also be best to try to I.D. the species you're getting just to be on the safe side + i dont know if it's illegal but I don't see why it would be, just dont steal anything super endangered......like..... a redwood or something ;p heheh.