Actually, Maeg didn't list all the contents of semen, an0n. If you'd like I can list for your the most common ingredients and the nutritional value if you'd like.
Regardless: there's a difference between bottled purified water and tap water, generally. I don' t know about your tap water, but the tap water we get in my area is chlorinated and flouridated and passed through rusty copper and iron pipes. The end result tastes something like licking a rock. Even purified water like Dasani (or Pepsi's slightly cheaper, but exactly the same product, Aquafina) is better than the tap water.
What I don't get is the percieved difference between spring water and purified water. Purified water has fewer impurities and has to meet higher standards than spring water, yet spring water is considered superior.