To me, there are 4 issues with ocean swimming:
1) rip currents - how to recognize the water patterns, what to do.
2) regular currents - how to pick a spot on the shore and stay lined up with it (a guard tower, a fluorescent umbrella, etc.), checking it every few "waves" and either swimming against the current to get back to your position or riding a wave in to shore and then walking to your location.
3) how to dive into a breaking wave - going under the crest and coming out behind it
4) how to go with the flow when you miss #2 and get hit by the wave. You have to know "you will always come up" so don't fight it.
An awful lot of this has to do with body strength and swim stroke strength. That's an acquired skill. Kids (and adults) don't realize how tired they get with ocean swimming. This is so very different from swimming in a pool where you are never more than 15 feet from a wall and where there is clear water to make you easily visible to a lifeguard.
Some ocean beaches allow those flotation suits that have inserts in the material, that the child actually wears. That helps keep a child buoyant without constant treading of water. No open water beaches around here allow strap on life jackets or "water wings" (nothing inflatable at all). Some of ours allow solid boogie boards which can help a child navigate on top of the waves, learn to paddle, help them learn the timing of riding a wave in to shore. Mostly, it's just experience.
Seriously, though, a child needs to swim laps in a pool (not just make it across once) or (better) do some distance-type swimming in calm but open salt water (harbor, bay) to get used to waves, salt, seaweed and sea critters. Otherwise, they aren't really safe in ocean water above the waist, since waves will put them over the head pretty easily.
I swam laps in a harbor when I was growing up (including some in a sweatshirt!) to build up arm strength. And a strong kick is important too - teach kids to kick without making a huge splash because it's much more efficient and will propel them more. Anything out of the water is pretty much wasted. Let them watch a swim meet on TV (or live) and note how little they see the swimmers' feet actually out of the water.