Wailord is one of the largest Pokemon. In fact it's 14.5 meters long and weighs 398kg, which makes it the tallest Pokemon, but not even close to the heaviest. For something that incredibly big it's actually very light. Wailord's pokedex doesn't mention any other measurements than height and weight so we can't get an exact three dimensional model of it.

But if we would assume that Wailord was a cylinder with a radius of 3 meters and a height of 14.5 meters it would get a volume of 409.98 m3. Rounded to 410 m3 (cubic meters) for simplicity's sake.

If we divide its weight with that volume.

398kg / 410 m3

It gets a density of 0.97 kg/m^3. Compare that to the density of air which is 1.225 kg/m3 and you'll see that Wailord actually is lighter than air.

This means that Wailord would float sort of like a helium balloon. The density of air is lower at high altitudes. According to this, the density of air at 2000 meters above sea level is approximately the same as the density of a Wailord. Poor Wailord being stuck up there.