Question

Why is shallow water more dense than deep water? Shouldn’t it be the opposite?

Answer 1

I would guess here but I think because of evaporation.

Explanation:

I think that shallow water is subjected to evaporation almost in its totality and tends to get denser almost uniformly as water evaporates and the concentration of carbonates/salts increases....

Deep water can mix letting the denser superficial portion (from evaporation) to sink and mix with the less dense below...

But mind you, this is only my guess….

Answer 2

I do not believe that to be true.

Explanation:

Interesting question. I thought water was totally incompressible (or perhaps I forgot anything I may have briefly learned about compressibility). Click on this link for a discussion on Wikipedia:
https://en.wikipedia.org/wiki/Properties_of_water

This Wikipedia webpage convinces me that water is compressed slightly under pressure. Scroll down to the Contents table and click the section on Compressibility. But what I see there does not suggest that the density decreases as pressure increases. I have to ask why you have the feeling that shallow water is more dense than deep water.

For density to decrease as pressure increases, volume would have to increase. Notice that it says

"As the pressure is increased, the compressibility decreases, being `3.9×10^"−10" Pa^"−1"` at 0 °C and 100 megapascals (1,000 bar)."

I believe that means that as pressure increases, the compressibility and volume decreases.

Any time it is compressed the least bit under higher pressures, that would mean that the density increases. If compressibility decreased to zero at some point as pressure increased, that would mean that volume no longer changed in response to the increasing pressure.

I hope this helps,
Steve