In the lesson, my answer was:
暑いので水をのみなきゃいけない。
But it is not accepted as valid and I can’t figure why.
Hey! 
The proper answer is のまなきゃいけない
By the way, なきゃ is contraction of なければ and なくちゃ of なくては.
Ouch, totally tripped on that ま. I should review this stuff after a good night of sleep before coming asking for help 
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Not a problem!
Have fun studying!
By the way, don’t you think the tip is too revealing for this one?
[casual colloquial] [ない + いく]