Neither is grounded TO the other.
The water main may - or may not - be the grounding electrode. You didn’t give us enough information to know for sure. Assuming that it IS the grounding electrode, the panel would be grounded at the water main.
For kicks, lets say the water “main” is plastic, the house is piped with copper or steel, but the pipes are in the attic and walls (therefore, they are not the grounding electrode).
In this case, the panel and the water main are “bonded” to eachother, by way of the conductor you spoke of. Assuming the neutral bus is properly bonded to the enclosure (also assuming this is the service panel), both the “water main” and the panel should be “bonded” to the grounding electrode, which “grounds” the electrical system. Got it?
Bonding is simply “joining” components to make them one. Think of bonding in terms of “epoxy” used to glue things together - attaching them to one another.
Grounding is simply providing a conductive path to ground/earth.
The metal piping and electrical system components are bonded (attached together) and grounded (provided with a path to ground/earth).