Seacocks- Marelon or Bronze
A couple of quick points:
Marelon, like bronze, needs to be kept lubricated. There are special lubes for the purpose and with lubrication the valves in question probably do not need replacement.
Bronze thru hulls when bonded can lead to damage to the glass in the vicinty of the thru-hull when struck by lightning.
Good quality bronze fittings are generally substanially more expensive than Marelon. Brass which is not suitable for below waterline use is generally less expensive than Marelon so be careful that you are not looking at Brass instead of bronze fittings.