OP, you have several Remington 870's right now, so in my opinion you'd be best just getting a longer barrel 28"-30" with a modified choke or choke tubes and go hunting. That's the "practical" way to do it. If you find you like waterfowl hunting there is plenty of opportunity to buy something else if you want to try a specialized semi. If you need an excuse to buy more guns, of course we understand completely are here to help!
I personally have a strong preference for pump guns for waterfowl. That's not a popular opinion, but having been around malfunctioning semis for many many years my preferences are based on some experience. No denying the gas operated semis are helpful for reducing recoil though.
My other advice is to not be duped into thinking 3-1/2" shells are any actual advantage. They are designed for American style competitive shooting on crowded public marshes where few opportunities and frustrated hunters leads to "hope" shots that are at extreme range. We don't need to emulate that kind of behaviour here. I don't even bother with 3" shells most times, if you do your job right with scouting and decoys, birds can be killed cleanly with good fast 2-3/4" steel shells.