I would always say "I am going in the water" or " Are you going in the water". That being said, I don't think Duo will ever accept " in the" for "ins". The translation for ins is "into the". It shows a destination from point A to point B. Furthermore, I believe if you ever see "in die" or "in der (masculine noun) , it will always be translated as "into the" as opposed to "in the" even if "in the" makes sense in English. If you see "im", "in den", or "in der (feminine noun) indicating the dative case it will be translated as "in the" as opposed to "into the".
I might be way off base with the preceeding opinion, but I will translate this example as "going in the water" or "walking into the water" in my mind, but I will respond to Duo with "going into the (ins) water".
Thanks mizinamo for your insite.
"Are you going in the water" should be accepted. Of course "in" in German means "into", but when speaking of water, what's the difference? You cannot go "in" water in any other way without going "into". Duo is splitting hairs on a bald man; there is no "there" there. Which is why British and American English would use "in" and "into" interchangeably in this context.