In Reply To
Plastic bag to cover the seat or handlebar area. That’s about all you can do.
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And make sure you dry off completely after the swim, because we all know that bike saddles aren’t designed to get wet at all. Oh, and try not to sweat while riding either. 
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I never understood the covering of the saddles. You might want to cover your bike computer, or remove it.
A wet saddle and wet bar tape has to add at least a couple of grams to the bike weight. That could be the difference between placing 1st and placing 3rd in your age group.