90mls in a baby that age is unlikely to cause water intoxication.

Don't fret about the water intake. Over the summer months we encouraged DD to have boiled water (she was a Feb baby) when she was on formula for 2 reasons;
1). extra hydration
2). to avoid constipation which can sometimes happen with formula fed babies.

But seriously, don't stress about it - just make sure you reiterate to your parents how to make the formula. Your parents may have just been confused and prefered to err on the side of caution and give plain water instead of risking "mixing it wrong".

MG