My youngest was on our boat at 5 days old and grew up on it. My next oldest (2) would rather be on the boat than his bike.
Do both. Teach them to swim AND do the sailing lessons. One does not preclude the other. As far as the tipping over or turtling, I agree. It will be an issue. If you plan to go ahead, just don't buy some piece of junk WalMart life jacket. Get a real childrens life jacket that works. Here are our personal requirements and the jacket we use:
1) Head float. It is a large piece of material behind the head that forces them head up and face up. What good is a life jacket if they can float face down????
2) Zipper AND snap. A good childs life jacket should have a full zipper and snap together to almost act as a suit.
3) "Crotch Strap". Sorry folks. I am sure there is some kind of technical term for this, but I don't know what it is. Basically it is a solid, snap in strap that goes from the back to the front (between the legs, so they cannot slide out of the jacket).
Our personal favorite: Lil Legends. You can see it on the West Marine web site. http://www.westmarine.com/webapp/wcs...01/12387/106/8
Sailnet also sells one but I am not familiar with it: http://shop.sailnet.com/product_info...ducts_id/14330
Now, all this being said, IT IS YOUR CHILD. IF YOU DO NOT WANT THEM TO DO IT, SAY NO. THese are just the opinions of various people and their experiences. Also, my boat(s) with the exception of our dink, were much bigger than this and WE were there, not someone else.