I'm an ok swimmer. I swam in the school swimming team, many moons ago. DH is also an ok swimmer. We'd happily swim a mile or so in a pool.
As others have also mentioned, swimming lessons for our children were non negotiable. They both got their 2k badges whilst at primary school. They are happy and confident in the water.
It's supposed to be in the national curriculum I believe, but we just filled in a form to say they could swim. I'm not sure what actually happened to those that couldn't.
Swimming lessons were eye-wateringly expensive for both of them. We couldn't afford them when they were younger and it was something that really bothered me. But they both did brilliantly even though they weren't swimming underwater at the age of 6 months
But, again repeating what others have said, the pool in Spain would have been quite cold as it wasn't heated. Cold water makes even the most confident swimmer panic. My brother has an unheated pool in his garden and my son had a real panic in it a couple of years ago in May. He could even stand up, but was thrashing around like a lunatic as the cold really took his breath away. We shouted at him to stand up, which he did, and we laugh about it now, but I think it taught him a valuable lesson. We even talked about it when we were walking up Southend pier last week. My mantra is to kick off your shoes, and float until help arrives. We again reminded them not to jump in after each other if there was a problem. It's a chat that we've had numerous times. But many families don't have the luxury of experience or swim sense, to have those discussions in the first place. My heart is absolutely breaking for that poor family in Spain
