If I don't drink enough water nor eat much, is it bad? How bad is it?

I'm 17 and I don't eat much like when I was 12-16, I'm like when I was younger... I didn't eat much and I was super skinny, but when I turned 12, I didn't live with my parents only my uncle, (he was almost never home kinda) It was boring and lonely, I got used to it and also ate a lot and got fat (average). then when I was 14, I struggled with the fat I still have (I weigh 112lbs (last time I checked), I'm 5'3" and before I was 80lbs (last time I checked when I was 12) and really skinny.) Now I eat less and sometimes I forget because I dnt get hungry, I only get hungry around food and when I really do get hungry.

People say I'm skinny, but I don't see it. especially when I have stretch marks from when I gained weight and I never lost it (they are not as bad now... I dnt know why or how its not like before...and that is good to me!)
If I don't drink enough water nor eat much, is it bad? How bad is it?
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