The weight of a substance is determined by - Its original weight - upthrust acting on it. In daily lives, we neglect the upthrust on us by the air because its negligible.
While a bucket of water is inside a well, it has upthrust acting on it due to the rest of the water present in the well.
So, its weight in water = Actual weight - upthrust
And its weight on land = Actual weight (upthrust is very negligible)
We can clearly see,
Actual weight -upthrust < Actual weight
Weight in water < Weight on land
Therefore, a bucket of water weighs less in well.