I grew up near the ocean (Atlantic Ocean) and we currently live near the Gulf so we spend a lot of time in the water. I have never put my kids in life jackets. My gut has always said that it seems less safe, because a child is more likely to go deeper out and because the jacket is pulling them up, they have less grip/strength in their feet to hold them in an upright position and are more likely to get knocked about by the waves.. I'm very strict about how deep I allow my children to go. In calm water (small waves) my 7 and 9 yo aren't allowed past their respective waists, and my 3-yo isn't allowed past his knees unless an adult is holding his hand at all times. If there are moderate waves (and we never get really big waves), the 7 and 9 year old aren't allowed past their knees and the 4-yo not past his ankles. We go to the ocean frequently and I grew up in the ocean, I feel that learning how to stand in waves, learning how to jump up and ride the wave and how to keep one's balance with the wave is important and since a life jacket increase buoyancy, I'm afraid it's going to throw balance off. I'm not a lifeguard or an expert or anything, this is just my gut feeling. I feel that having a safe, strong grip on the ocean floor while standing, staying in shallow water and being familiar with waves and how to ride them, and staying out of water known for riptides or strong currents is the most important factor for safety. Where we live now, there is one beach which is known for having sneaky riptides and at the beach, swimming is prohibited. Where I grew up, there was one beach that everyone *knew* was dangerous, and we never went there. So, I feel like generally it is "known" where the dangerous beaches are, so doing research before going and swimming is also important. This is for the Atlantic and Gulf anyway. I've never been to the Pacific..I think the Pacific generally has higher waves and stronger tides, so that could be different.
In all of my childhood swimming in the ocean, I can only remember being knocked down by waves a few times, and then it wasn't a big issue to find my feet and stand back up again. As soon as the wave passed, I quickly was able to get upright again and stand up, so I never felt in any great danger. But, I also never went out very deep and was always cautious in that regard.
Now, if we my kids were ever on boat or anything, they would definitely wear life jackets.