They can often be used to wash yourself and others. Or you can use them to get water for nefarious and other purposes.
For the water skill, see magic.methods.elemental.water.
kneel and all the other lovely positions in a bath. You cannot
sit etc in a washbasin.
empty baths and basins.
wash yourself and others in baths and basins, and you can also
Remember if you
wash in a bath or basin, the water will get dirty and can't be used for coffee machines, marbling trays, brewing, or other watery needs.
You can sort by the
Liquidcolumn in the table in Furniture#Storage_furniture to see all the water furniture items.