A leather collar that is repeatedly water saturated will deteriorate faster than a collar that is not - salt water being particularly damaging - and therefore we would always advise against using a leather collar when swimming.
We also understand that dogs are dogs!
Therefore, to help with the longevity of your leather collar allow it to air dry (not on a radiator) after it gets wet ensuring complete dryness before using again. Then treat with a leather restorative, such as saddle soap or Leather Genie (beeswax product) to nourish the leather and protect against the elements. These products may alter the leather colours.
Do not allow the leather to come into contact with chemicals from commercially available flea spot-on treatments. These will damage to the leather and may cause the dye to run.