The through hulls are 20 years old and they're nylon. They all need to be replaced. They break because nylon breaks down in UV. IF one snapped, then they are all ready to snap. To remove mine, I stick a flathead screw driver inside (from the outside) and apply pressure up or down. You would be surprised how brittle they are.
I'd replace with either Marelon or Bronze. Stainless Steel only works with oxygen. Water and the absence of oxygen will rust SS and make them as unreliable as nylon. If cost is an issue, then go with Marelon, which is UV resistant.