If you purchase an iWatch, do you have to add an additional line on your bill?

To be clear, a GPS only Apple Watch does not have any extra fees, you cellular carrier is not involved, and the initial cost is approx $100 less. The GPS only model can do Everything the cellular model can do as long as your iPhone is nearby,w here nearby is somewhere within say a 3-4 bedroom home.

The cellular model, if you activate the cellular radio, shares your iPhone's phone number for a monthly fee, as explained by fieryjack

No what happens is you pay an extra monthly fee on your existing phone contract, the watch uses your existing phone number to make calls in the US it's about $10 per month,that is for the cellular version of the watch, if you buy the GPS only version of the watch you can make calls and send texts as long as you have your phone with you,with the cellular version of the watch you can make calls and send texts without having your phone with you.So in reality your paying $10 a month so that you can leave your phone at home.