metric is very conveinient
1gram=1cm^3 of water
1m^3=1,000,000,000 grams
water is 18.015 grams per mole
(1,000,000 grams water)/(18.015 grams/mole) = 55509.3 moles
there are 6.02e23 molecules in 1 mole of water so
55509.3 moles * (6.0221E23) = 3.3428E27 molecules of water in 1m^3 of water
so we have200 of those
200 times 3.3428E27=6.6856E29 molecules
answer is 6.6856 times 10^29 mlecules