I just regreased the o-rings on my water
caps. If these leak you will have sea water
in your fresh water and fuel
along with addition fresh water from incidental rain.
Take the o-ring off the cap and inspect it for rotting and shredding. Shredding is from tightening dry. Rot
can be detected by stretching the o-ring and looking for cracks.
Also, look to make sure the cross-section of the o-ring is not flattened.
Replace as needed. Coat with a thin layer of good silicon o-ring grease to lubricate and to maintain a water proof seal.
I have personally observed many boats where the o-rings are gone, totally rotted, and/or dry.