If European Sea Ducks are anything like American Sea Ducks, I wouldn't try them with anything less that a 12 ga with tungsten matrix (my personal preference to Bismuth). These birds teach Canada Geese about tough in their spare time.
Salt water is an even greater equalizer than Col. Colt's pistol; it reduces everything to rust given any chance. If they allow a plugged pump gun or auto, I would go with a Remington 870 or 1100 (a layout boat may not be the best environment if you are not used to a pump) and you can take it apart and wash the salt out down to the last bolt, pin, and/or washer. For doubles, a Stevens 311 worked well for me, and I used a lot of WD-40 before it became unfashionable, and it had no rust on it and functioned perfectly when I finally sold it. If you are an O/U rather than a SxS guy, I can't think of a better American candidate than a Red Label.
I would concentrate on a shotgun you are comfortable with and borrow a rifle if the opportunity presents.
Hope you have a great time.
