I am not sure of the name, But I've heard them called 'water turkey'
penguin
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.
No bird can fly underwater, it would be swimming. An example would be a duck.
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
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.
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