The answer to this has to be weighed in light of the revision of the CGL form in December 2004.

If the insured is covered under the pre-2004 CGL form, we would find that the water truck as you describe it would fit the definition of mobile equipment under the CGL form. There are at least two portions of the definition that would apply.

First, the truck is a vehicle the insured maintains primarily for purposes other than the transportation of persons or cargo, that is, to help in his construction business. This is what this portion of the mobile equipment definition is intended to cover—a piece of equipment used in the business of the insured where the main liability exposure is on the construction site, not on a road used by the general public.

The second part of the definition of mobile equipment that is applicable here is that part dealing with self-propelled vehicles with permanently attached equipment. That part states that vehicles with permanently attached equipment designed primarily for road maintenance, but not construction or resurfacing are considered autos. Your insured's water truck is used in construction, not road maintenance, so it is clear that the definition of mobile equipment encompasses the water truck.

Now, if the insured has the December 2004 CGL form, the definition of mobile equipment has been changed. Mobile equipment does not include any land vehicle that is subject to a compulsory or financial responsibility law or other motor vehicle insurance law in the state where it is licensed or principally garaged. If the insured's water truck is subject to a motor vehicle insurance law, it is considered an auto and the liability coverage must be placed under an auto policy. You need to check with the financial responsibility or motor vehicle insurance laws in your state to confirm the status of this water truck.