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?
emu, it may not fly but it is bretty dam big ( knowing this I'm from AUS)
dove
albatross
seagull
sparrow
robin red breast
crow
blackbird
Perhaps a Pelican or a Swan.
An ostrich.
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.
Probably an Albatross.
Gannet
Blue bird
Albatross
The bird described is a coot. There are 12 extant (living) species of coot.
It is a large sea bird that is often white with black at the end of their wings.
The penguin is an aquatic bird. The name contains nine letters.