consult a lawyer/survivor.
If the land behind you is trying to get a water main extension to sell land for a development, ask were the development is going to get its access to the "main road"
when they say "over there" then thats were you say put your water.
If they have "driveway" acess to the property and are considering building a house back there and want a "water service" through your property, you should be able to write other conditions along with the easement, ie its never to be used for a "road".
If this is on an existing property line, then remember even if you only sell 8' of ground to him, there is nothing that keeps your neighbor from selling him another 22' of HIS property, enough to create a "road" through.
If it was my ground, i wouldn't do it period under any circumstance.