There are tons of good semi-auto's out there. Browning, Barretta, Franchi (my dad has one of these and loves it), Remington and even Winchester. Each has its own fans, but you really can't go wrong with any of them. For a pump, I would suggest the Remington 870. They have been around forever and are hard to beat.
I hunt upland game so I don't own a duck gun (I have Belgein Browning Superposed in 20ga), but if I was going to buy one to carry around in a boat or in a blind in crappy conditions, I would probably by a 3 inch Magnum 870.
Here you can buy a plain one at Wal-Mart for under $300.00.