I read somewhere, on this list I think, that someone had one of those “plastic” thru hulls for the sink drain and discovered that it cracked and was leaking. So I guess it would be advisable for you to change that thru hull. However, I would use a bronze thru hull with a Marelon ball valve. However, there are people who do not like the Marelon valves as they have a tendency to have their handles break off. However, with proper seasonal greasing, they should operate faultlessly for many years as mine have for twelve years. A second issue is that many of the small ½ inch to 3/4 inch vales are brass and not bronze. Placing a brass valve onto a bronze thru hull is a recipe for disaster in that the electrolysis that sets up between the brass and the bronze will destroy the brass valve which will then fail and leak badly into the boat. Marlon on the other hand has no electrolysis complications with bronze thru hulls and hence is a better selection over a metal valve in that you cannot be sure if the metal valve is really bronze or actually brass with in incorrect label of bronze placed onto it. The intake for the toilet is probably best just left alone, as they say, if it an’t broke, don’t fix it. Holding tanks are still a viable option on a C-22, and while such may not be to your particular needs, it may be important to a respective purchaser of your vessel in the future.