Bronze is not an opinion based on a whim, it's a proven alloy for such things as through hull fittings and marine valves.   There is a thing called exercising your valves .  Opening and closing them helps to keep them free and working.  it's leaving them in one place for like ever when they seize.   I try to open and close my seacocks monthly.   you can aide in their life if you can run some freshwater through them at a lake or river.  
oh, when underway close your through hull fittings like the washdown and livewell ,if you are not using it, because many a boat have succumbed to a failed fitting or or hose while underway and the captain often doesn't detect it until the hull is handling poorly. also , if you keep your boat in the water , according to Boat US, the single biggest insurance claim is sinking at the dock and the culprit is often an open seacock and poorly maintained hoses that fail.