>Everyone's holidays are at a beach, so water safety is a constant concern

That explains the discrepancy just fine to me. Most families in the US who expect to be around water a lot get swimming lessons for their kids, and most families who don't, don't. I had swimming lessons growing up and mostly they just reaffirmed my belief that I hate the beach more than life itself and will never, ever, ever step foot in nature's latrine.

A ballpark estimate for Portugal is that it costs the government probably 50 euros-ish per pupil for basic swimming lessons? Given that they cut drowning deaths by almost an order of magnitude in children, that seems like pretty good ROI.