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.
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