The duck is swimming IN the river or
The duck is swimming ON the river?
Here i got question, we say "we swim in the river, the fish swim in the river", but what about DUCK ?The duck is swimming IN the river or The duck is swimming ON the river?Thanks for your help!

Question

The duck is swimming IN the river or 
The duck is swimming ON the river?
Here i got question, we say "we swim in the river, the fish swim in the river", but what about DUCK ?The duck is swimming IN the river or The duck is swimming ON the river?Thanks for your help!

Answer

I'm also going to disagree with Melinde-Mari.

Fish swim in rivers.  Ducks swim on rivers.

Answer

It will still be "in" the river

Because the duck is swimming so it inclines that something in this case the duck is partially or fully 'in' the water also because you're telling us 'where' the duck is swimming. With "Swimming" it will always be "in" that is used, but ~

You could say the duck "float's on water" but as soon as saying 'where' it'll change to "in"
"The duck is floating in the river... on the water..."

Answer

I would say "on" as it implies on top as ducks swim on the surface. "In" implies underneath the surface like another answer said like fish.

Answer

I'm going to disagree with Melinde and go with "on", especially as ducks swim on the river but I can also imagine "in" would work well.

So both....

Answer

You could use either but the more correct expression is swim IN the water. Also, float ON the water.  The duck swims in the river. The duck floats on the river.