There are many large white birds on the planet ... you'll need a better description. How large is it? What is the wingspan or height? What other markings does it have? What color are the bill and the feet? Which part of the world does it live in?
A swan is an aquatic bird with white feathers
A possible word is pelican, a large aquatic bird.
An aquatic bird is basically called an aquatic bird.
A swan
A penguin may be the most suitable answer, but a loon is a consideration.
Gannet
Probably an Albatross.
Blue bird
Albatross
It is a large sea bird that is often white with black at the end of their wings.
black vulture
The bird described is a coot. There are 12 extant (living) species of coot.