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All of them. Whatever units a country uses are 'customary' in that country.
Lawri unit
No. I would use linear units if measuring the swim distance or cubic units if measuring the volume of water. Square units might be useful to see how many people could use the pool before it got crowded but that would depend on whether they were there for swimming or splashing about.
You always use square units when measuring area.
Lots, but most people use only the Metric System.