The pools at Mandalay attract a large crowd at pretty much any time of day, and a significant amount of people arrive right when everything opens in order to secure seats. Our first time exploring the wave pool, regular pool, and lazy river left us searching for seats for 15 minutes, eventually finding two on the end of the lazy river next to the hut where they rent / blow up tubes. Yay for seats, but not super relaxing.
The Delano Beach Club, on the other hand, always had seats available. The vibe was considerably more laid back, and the pool was much less crowded. I was able to get a seat here each time I went, even at consistently different times of the day.
The seats themselves were newer than most of the rest found throughout the Mandalay property, and were definitely more comfortable. Waitstaff were consistently circulating, taking drink and food orders, and there was free water and bathrooms within 25 steps of just about every seat.
You do have the opportunity to reserve a "choice" lounge chair or cabana, but if you don't want to spend anything extra, there isn't really any need to. There were open ones every time I was down there.
This, however, is a summer review - given it was 110 outside, the heat might have thinned the crowds a bit. If you are coming when it's a little cooler...it might be a little more crowded / harder to find a seat, but I don't think it would be any louder or less relaxing than it was for us.
Any way you look at it, you'll likely get a chair, there will be sun, and you'll have no less than two giant, golden buildings to stare at and wonder what the penthouse suites must look like.