I grew up swimming in lakes and in the ocean. Â I then became a lifeguard and WSI as a young adult and adult. Lakes and other natural bodies of water are entirely different settings than a pool, a person can disappear into the murk very quickly. Â Drowning can look like play initially, or it can be very very quiet. Â
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I will share  our family rules with you- they work for my comfort level, but that also hinges on my being a very strong swimmer who is pretty confident about being able to effect rescue in some pretty tough conditions.Â
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1. Kids who don't swim well use a PFD unless they are in arms. (This is my two little guys, they are 1.5 and nearly 3.)
2. My daughter (age 9) is not allowed past chest level without a buddy, and both she and the buddy have to have proven themselves to be strong swimmers. (A 500 yard swim (accompanied) and 10 minute water tread test is part of our annual water readiness testing for her.) She also has to inform me when she wants to go out deeper and can only do so when there is another adult with us to watch the littles so I can focus entirely on the bigger kids in deeper water. Â They don't always want me right there, but I still will have eyes on them the whole time and will keep them within a distance I can comfortably cover with some speed. Â I know the bodies of water in which we swim, and I won't let her go much deeper than about 10- 15 feet without me right being right there (that's the limit of depth I am comfortable diving to to effect rescue. Â For those who don't swim often, this will be much more shallow- anything beyond 6 feet tends to begin to hurt your ears and can be very hard to reach without a lot of practice.)
3. If the water has a current (ocean, or a lake with strong underwater currents) I am more strict about her being closer to me or she can elect to wear a pfd.Â
4. At any depth she's swimming in on her own, she has to be able to prove to me that she can descend to the bottom (pencil dive) and push herself back up to the surface without much difficulty.Â
5. I have a whistle with me. Â If it is blown they have to immediately get out of the water and sit on the shore. Â Even my 1.5 year old has this mastered this year. Â
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In pools, I don't encourage PFDs. Even with my youngest, I want him to learn what happens when he falls into the water face first. Â I want him to learn to stand back up instead of being passive and being floated to the top.Â
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Now, all of that is to say that I am probably more strict than many parents are, but I will also allow a great deal of freedom as my child advances in skill level.  My daughter passed her level 5 swim test last year, and is working towards level 6 at this point.  Most kids her age are at level 3/4.  You can find the levels here http://www.ci.berkeley.ca.us/uploadedFiles/Parks_Rec_Waterfront/Level_3__-General/AmericanRedCrossSwimLevelRequirements.pdf and think about where your child falls as you assess what you are comfortable with and what you might want to work towards as skills.Â
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I come from a family of competitive swimmers, and my children grow up in water from the time they can crawl. Â DH does not swim as well. Â He does not take the kids swimming alone, and certainly isn't going to be the one helping our oldest. Â He can help the littles as they are still wading and playing only in shallow water. Â If he did take my daughter swimming alone, she would not be allowed out as far as she is when I am with her.Â