Recently I have been sweating buckets. Maybe you can imagine already that this idiom means to sweat, A LOT!
Buckets can be used to collect water, and often in an emergency. When a boat (a small boat) is filling with water this is a problem, the boat could sink. We use a small bucket to take the water out of the boat.
Another problem is if we have a leak in our roof or ceiling and water is dripping down on the floor. When this happens we usually use a bucket to collect the water.
If we need a 'bucket' there is too much water. If we are sweating a lot the feeling is not good, and we are uncomfortable. We use 'bucket' to mean it's too much.
"It's so hot today! I'm sweating buckets!!!!!"