YOU SAID:

I've been drinking watermelon.

INTO JAPANESE

私はスイカを飲んでいた。

BACK INTO ENGLISH

I was drinking watermelon.

INTO JAPANESE

スイカを飲んでいた。

BACK INTO ENGLISH

The watermelon was drinking.

INTO JAPANESE

スイカを飲んでいた。

BACK INTO ENGLISH

The watermelon was drinking.