An interesting little question and a fun bit of maths...
The molecular weight of water is: 18.02 g/mol
The density of water is 1 g/ml
Therefore 1 litre of water (1000 ml) would weigh 1000 g
The molarity of something is the number of moles of that thing per 1000 ml volume.
So we have 1000 g of water in 1 litre, and the molecular weight is 18.02 g/mol
Hence mass divided by molecular weight gives us the number of moles, so 1000 / 18.02 = 55.49 moles.
The 1000 g of water is in 1000 ml (1 litre), so the molarity of water is 55.49 M
If you are struggling with moles and molarity, you might find this blog post useful: Why is memorising the molarity formula a bad idea?
You can also test your understanding of the above calculations at: Maths4Biosciences.com.