pool thermometer? should do the trick
I tried this many years ago.
Problem is you are often looking for temperature breaks of only a few degrees or so.
As you lower or raise a pool thermometer through "other" temperature waters it starts to record that.
So you drop it to 25'.
Let it sit there and it's let's say 50F.
As you raise it to the surface, which is at 72F, the pool thermometer goes to the 72F mark somewhere.
Hard to get readings at various depths accurately.
If you simply want an idea of water temp at or near the surface, ok, but he asked for various depths.