Most people are pleased to get advice when you explain what they are doing will cost them dollars down the track.
I saw two young guys filling the hulls of their cat with water. I couldn't help myself and wandered over to ask why. They said they were leaving it on the beach and water ballast meant it wouldn't blow over. When told about osmosis they pulled the bungs, dried it out and tied it to a tree.
Sometimes though others won't take advice if you hit them with a fencepost. New owner of old cat enlists our help to set up on beach and then asks how to sail it as he hadn't had a prepurchase test sail. We said go out 200 yards and come back, then do it again. Stay close to shore so if you get in trouble someone can assist you. We said we will watch you. A short time later he disappears around the point of the bay as the breeze fills in apparently racing another cat. Hours later people start leaving the beach but we walk to the point and see the cat on its side drifting further offshore. I convince a returning fisherman in a tinny to head out and we eventually find the cat sailor sitting on one hull, a big lump on his forehead and being violently sick. He has no idea or the strength to get cat upright or to keep it upright in the breeze, or get back to shore with one rudder and the rudder bar torn off. We get cat upright and tow him in, pack up his gear and he disappears. Soon after there is a For Sale advert in the paper. I bet he's never gone near a cat since and tells his friend 'They're dangerous and unpredictable, look what happened to me!