Brno ZP49, definitely. Light, excellent handling, available with either straight or pistol grips, beavertail or splinter forearms. A very classy, classic SxS with a "British" feel to it. They come up on the used market regularly, usually for under $500. And, in case you care...it's an actual sidelock gun. Check around to see what a new sidelock gun from any manufacturer will sell for these days.
Downsides: you should probably use bismuth or other non-toxic shot (i.e. expensive!) rather than steel for waterfowling. Also, they have fixed chokes, and a couple of gunsmiths told me that they would not thread the barrels of the ZP49 for chokes due to the thin walls. The one I had in modified and full chokes worked beautifully for ducks and even a couple of turkeys. My current one was reamed out to improved cyl and modified and is the best upland gun in the world!
The other potential complaint (not really a problem, more a marketing-induced mass delusion) is the 2 3/4-inch chamber. Yes, it hasn't the payload of a 3- or 3 1/2-inch gun...but it is just as capable within its limits as a .308 used for moose instead of a .300mag or .338. Check one out it you get the chance. It sounds like exactly what you are looking for.