Buying liquidand ordering drinks and sweets

"Liquids" is okay but a little strange. You can be more specific and say "drinks", since the context is drinks at a restaurant.

IfWhen you are in a restaurant, can you say these phrases to a waitperson?:

Note: "waitperson" is not commonly used. It's still a real word, but another way to say this could be "waiter or waitress", or just "server".

B. I'll have a water.

no, don't say this

Does "I'll have a water" mean a glass of water, a bottle of water, a jug of water or can it mean all of these?

Don't say "I'll have a water." You should say:
"I'll have water."
"I'll have some water."
"Can I please have some water?"
"I'll have a glass of water."

Your server asks: "What would you like to drink?"
Response (simple): "I'll have water." "I'll have juice." "I'll have iced tea."
The server will give you your drink in a cup / glass / bottle, or whatever they have!

Response (with measure word): "I'll have a glass of water." "I'll have a cup of tea."

You don't need to have a measure word when you are asking for a drink in a restaurant.

If I ask for 2 colas, does it mean for 2 bottles of colas or 2 glasses of cola?

I would say "Can I have 2 bottles (or glasses) of cola?"

-half a kilo of frutti di mare sweets/candy,

yes

- two milks (meaning 2 cartons of milk),

no, say "two cartons of milk", "two bottles of milk"

- a citrus scent perfume (meaning a bottle of perfume),

yes, this is fine

-2 white chocolates (meaning 2 bars of white chocolate)?

I would say "2 bars of white chocolate". However, maybe you're at a candy shop and you're picking many different kinds of chocolate bars - then if you say "I would like to have 2 white chocolates, 3 dark chocolates, and 1 mint chocolate", that is perfectly okay!