I did NCS in 2016 and I was terrified of deep water (still am).
Though I ticked the 'Can't swim' option, when we went rock climbing, we were required to jump into the water and then 'swim' to get back onto the rock. My instructor claimed there was no other way to get back up.
I repeatedly told her I couldn't swim but she assured me I was wearing a life jacket and everyone was around me so it was fine. I was ******** my pants but she kept telling me to jump because I was holding everyone up blah blah, so I did it.
Long story short, I had a panic attack as soon as I hit the water and couldn't keep upright. She (my instructor) started panicking right about then because I couldn't breathe but I made it out alive so it's all good!
I'm not telling you this to put you off, just making sure you're aware. I would say be adamant from the start that you can't partake in the swimming activity and just refuse to do it if you know you're going to panic as soon as you hit the water. However, if the problem is you just can't swim then tell them about that and I'm sure it'll be fine. For me, it was more of an issue because of the whole fear aspect which led to a panic attack.
We did another water based activity after this and my mentor, who was probably made aware of the situation, sat in front of me and just kept an eye out which was super helpful.
Sign up, and just go for it! It won't change your life but it will be a fun summer.