It's similar to one aspirin vs 10000. Also, people do thumbprints and live normal lives after, I just threw that out as an extreme. LSD is very safe if you're doing normal doses. Most people don't take 10000 doses so I can't really speak to that.
The mechanism for water toxicity is physically displacing ions in the brain, causing osmotic damage. (This also explains why lower amounts of water are safe.) A thumbprint dose of LSD is nowhere near high enough to do something like that.