对不对

你是喝茶啊 还是喝水呢？
Do i need the 是 after ni?

Correction:你喝茶还是喝水？

对不对

你是喝茶啊，还是喝水呢？

Do i need the 是 after ni?

“你是喝茶啊，还是喝水呢？”and “你喝茶啊，还是喝水？”both OK.

对不对

你是要喝茶啊 呢 还是要喝水呢？
Do i need the 是 after ni?  Yes,  and 要 also. "Do you want to drink tea or just water?"

你是喝茶呢还是喝水呢？ 

你喝茶还是喝水？

你是要喝茶还是要喝水？

The sentencees above are all right.

The 是 after 你 is optional. But the sentence doesn't sound very natural unless  啊 is removed or replaced by 呢.