Native speakers will say like this: “Feel free to drink some water”.
There are lots of other ways to say it, but this one is the most accurate and most commonly used.
Here is an example on how to use this in a conversation:
A: My throat is really sore.
B: Feel free to drink some water!
ネイティブスピーカーは次のように言います。
“Feel free to drink some water”（自由に水を飲んでください。）
他にもいろいろな言い方ができますが、これが最も正確で一般的です。
以下は会話での使用例です。
A: My throat is really sore.
喉がすごく痛い。
B: Feel free to drink some water!
自由に水を飲んでくださいね！
1.
"Have" is usually used to expression possession, but can also be used to express the action of using/drinking/eating and more. For example, "Have a burger."
2.
I suggested the word "something" which can be used to express any substance or liquid, and not only the specific, such as water.
3.
You can use "feel free" to express that you do not oppose an action.
For example, "Feel free to visit me" or "feel free to use my telephone."
1.
"Have" は、通常、所有を表しますが、飲む/食べる/使うなどの意味でも使うことが出来ます。
例えば："Have a burger"（ハンバーガーを食べる）
2.
"something" という言葉を提案しました。"something" は、何か（例えば「水」など）に特定することなく、どんなもの（液体）にも使えます。
3.
"feel free" を使って「ご自由にどうぞ」と言うことが出来ます。
【例文】
"Feel free to visit me"
［訳］ご自由に訪ねてきてくださいね
"Feel free to use my telephone."
［訳］ご自由に私の電話を使ってくださいね
When you tell someone to "go ahead," you are giving them permission to do an action. In this case, it would be to drink water. This is a fairly informal way to give permission but clearly understood by native speaker. I hope that this helps! :)
If you want, you're welcome to get/take a drink of water..
Feel free to have drink of water.
If you need to take a moment to get a drink of water, its no problem.
Using phrases like, "feel free to...", "Its no problem, if...", or "You're are welcome to...." to state that it would not be a problem to do something or some kind of action.
次のフレーズを使って、「ご自由にどうぞ」、「問題ないです」と伝えることが出来ます。
"Feel free to..."（自由に～してください）
"It's no problem, if..."（～しても、問題ないです）
"You're are welcome to...." （～しても、いいです）
Please drink water if you need some. I don't mind.
Feel free to get a drink.
mind - be offended; care; object to; disapprove of
The phrase "I don't mind" is used to show that you are not bothered by something. For example, you would not be offended if your teacher got a drink of water.
feel free - do not hesitate
mind - 気に障る / 気にする / 反対する / 嫌だと思う
"I don't mind" は「～を気にしない」「嫌だと思わない」という意味です。例えば、先生が水を飲んでも嫌な気持ちにはならない、ということ。
feel free - 遠慮しない
If you would like to express that you don't mind your teacher taking a moment to have a sip of some liquids, use the sentences above.
This act displays great manners and the teacher will feel glad at your offer.
"Don't mind me, please have something to drink teacher."
"Sorry teacher, are you feeling okay?"
「自由に飲み物を飲んでください」と先生に伝えたいなら、上記の文が使えます。
これはすごく気遣いのある行動ですね。先生もこのように言われるとうれしいと思います。
"Don't mind me, please have something to drink teacher."（気にせず飲み物を飲んでください、先生）
"Sorry teacher, are you feeling okay?"（体調は大丈夫ですか、先生）
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南アフリカ 
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