The most common duck in the Gainsborough area is the humble Mallard. This is the duck you are most likely to see on the Trent all year round.
At low tide they will congregate around Morton Turn, especially in the winter months. I counted 75 there on Oct 1st 2017.
Mallard (male and female) (picture from Wikipedia)
Beckingham Marsh is the best place for waterfowl in this area, and in winter there are sizable flocks of Mallard, Wigeon and Teal. Shoveler can be seen there all year round, and I have occasionally seen Gadwall there as well, and a single Pintail on Mar 11th 2018. The warden there has reported seeing Tufted Duck there recently, but I missed those.
Left to right: Wigeon, Teal, Shoveler, Gadwall (pictures from Wikipedia)
Shelducks are often seen at Beckingham Marsh as well, usually one or two, but there were several there on Mar 11th 2018.
Shelduck (picture from Wikipedia)
On the Trent you will generally only see Mallard, but just occasionally something else will turn up. I have seen the odd Wigeon or Teal amongst the Mallard at Morton Turn, and there was a single male Tufted Duck there on Feb 16th 2017, and a single male Pochard on Nov 16th 2018.
male Tufted Duck (picture from Wikipedia)
On Dec 12th 2012 there were 8 Goosander (5 males, 3 females) on the river at Gainsborough. I saw them again the next day on the river at West Stockwith, and the day after the website for Idle Valley (near Retford) was reporting they had them.
I saw Goosander again four months later, on Apr 1st 2013, 2 males and 2 females on the river between Morton and Walkerith.
male and female Goosander (pictures from Wikipedia)
There are often 1 or 2 Greylag Geese present at Beckingham Marsh on any day, exceptionally there were 11 there on May 2nd 2017.
I have occasionally seen one or two on the Trent at Morton Turn, and two flew over Gainsborough on April 20th 2018.
Greylag Geese (pictures from Wikipedia)
The warden at Beckingham Marsh has reported seeing Pink-footed Geese flying over occasionally, with two actually landing there in March 2018. I saw two fly over the hide there on Apr 20th 2018.
Pink-footed Geese (pictures from Wikipedia)
Flocks of Canada Geese can sometimes be seen flying high over Gainsborough, and sometimes in large numbers. The highest number I have a record for is about 180 over Gainsborough on Oct 15th 2017.
A few occasionally land at Beckingham Marsh.
Canada Geese (pictures from Wikipedia)
The warden at Beckingham Marsh reported seeing a Barnacle Goose in March 2018.
Barnacle Goose (picture from Wikipedia)
One or two Mute Swans can occasionally be seen on the Trent at Gainsborough or Morton.
Mute Swans sometimes turn up at Beckingham Marsh, usually one or two, but there were 10 there on May 11th 2018.
Mute Swan (picture from Wikipedia)
The warden at Beckingham Marsh has reported Whooper Swans flying over there, though I have never seen them in this area.
Whooper Swans (picture from Wikipedia)