I am not sure of the name, But I've heard them called 'water turkey'
penguin
An eagle flies and is a bird, a seal swims in arctic areas and is a mammal.
Probably a member of the Cormorant family. These are fish eating birds well known for their underwater swimming abilities. Loons are a member of this family.
it is a bird that swims on water such as a swan or duck.
A penguin.
A bird that swims in the water, so I suppose you could say either bird or both.
This is not enough information to identify the bird. You need to see it when it is above water level! I have seen these, and believe you might be referring to what is commonly called a Water Turkey.
Bird-That-Swims - A bird that doesn't fly, but swims Cowboy-Who-Rides-An-Ostrich Boat-That-Can't Stay Afloat Spotted Elephant Charley-In-The-Box Misfit Dolly
A bird flies because of a variety of specialized adaptations. THAT IS WHY A BIRD FLIES!
There is no underwater bird specified for Illinois. The official bird of Illinois is the cardinal.
Angry birds underwater
No bird can fly underwater, it would be swimming. An example would be a duck.