... You can imagine night time, with the border line between sea and sky not clear at horizon.
Same is the case in bad weather as well....
Point is, the pilot becomes unsure of his aircraft position relative to ground.....
Pilots are therefore trained to trust their instruments at all costs in such conditions. But then, you pay the price if you don't.
I still remember, that JF17 over Arabian Sea, was flying normally, but once it crashed, the altitude loss rate was horrendous enough..... thousands of feet in a few seconds, almost went nose down....
No may day call, nothing.