For one thing, it has to be in plain site to use it...and you'd use it every time you leave the boat. For another, the seacock would have to be closed to use it, so there's no exposure to sea water except when you're aboard to notice any problems. And finally...although some garden hose y-valves are metal, all mine are plastic.

So it's just as safe as any other plastic y-valve or shut-off valve in a line connected to a below waterline thru-hull...maybe even safer than a lot of connections in bilges that owners never even think about, never mind inspect regularly.