Salt concentration increases the density of water
Salt concentration increases the density of water. However, the typical pool (even salt-water pools) will have a lower concentration of salt than most seas. Therefore, seawater is typically more dense than pool water, and is easier to float in.
I think your thinking of the concentration of salt in the water and that would depend on if the water was filtered or not when you fill your pool compared to the sea.I would say the sea has more salt.