Yes the word file is countable. For example you can say: We have many files. or We have one file. Therefore it's countable. The noun water on the other hand isn't countable because you can't drink many waters. You can drink many bottles of water or drops of water, but water by itself isn't countable.
Yes, the word 'sea' is a countable noun. The noun 'sea' is a word for a body of water, there are many seas in the world.
countable
countable
The noun 'hill' is a countable noun. The plural form is 'hills'.
cookies are countable unless you have brain problems
countable
Prawn - prawns is the plural - is a countable noun
Shark is a countable noun.
Duck as an animal is countable, but if you mean the meat it is uncountable.
Countable
few is countable