What's the deal with this? I've never understood it.
If you go away on holiday, chances are the pool won't be heated because that costs a lot of money to do so. But I've yet to use a pool on holiday that's ever been too cold to be in.
Yet I've heard/read so many complaints about it, literally in real life (usually whilst I am swimming in the pool) or people complaining in Tripadvisor reviews.
People dip their feet in and go "Ooooh it's so cold" or they get in for 10 seconds and then scarper because they can't hack it, when all they need to do is swim around for 30 seconds to acclimate to the temperature of the pool.
I'm sure if you are in Iceland in winter, or even in europe and the outside temperature is 5°C or something then the pool will be bloody cold, but I can't say I've ever wanted to use a holiday pool in that sort of weather anyway. But when the suns been on it and it's 20+ °C there's no such thing as a pool that's too cold IMO.
I think people expect the pool to be "comfortable", not "warm" and as several people have said different people have different levels of tolerance for the cold, ranging from the northerners who'll go swimming in the sea dodging icebergs (preferable to dodging the brownbergs we're getting now tbh), to those that can't really stand it being below a reasonably high temperature, and then there are the elderly etc who might feel the cold more anyway.
One of the best ways to deal with this without artificial heating (or with minimal heating) is to make the pool larger but with an average depth that is quite low and then paint it so that it naturally absorbs more heat from the sun.
Wallingford on Thames has/had quite a nicely done outdoor pool*, basically you had the normal size pool but it was effectively heated by a very large (from memory about twice the surface area) paddling pool next to it that was only 15-30cm deep, with the water being pumped between the two (and I suspect filtered on it's way back to the main one, as IIRC it had a shallow "waterfall"/slope from the higher main pool to the paddling pool).
Also you're forgetting that 20c sun takes time to heat water, how long that is depends on the depth of the water and the volume, 20c on a normal pool is going to take a fairly long time to warm it noticeably if it's been cold over night, especially if it's not been particularly sunny the day before, but if you design the pool intelligently you can massively speed up its natural heating, but even then it takes time so if you go out at say 10am the air might be 20c, but the water in the pool might have cooled right down overnight to near the night time temp (and in the last week or two here that's been as low as something like 10c).
*We used to go camping there when I was a kid, and whilst the pool could be quite cool it warmed up quite quickly on sunny days due to the design with it's very large effective surface area compared to the volume of water.