xnmr53,
I just used my new fancy GPS on my quad a couple months ago on the trails in southwest Utah. The same terain as we have here in Nevada. I Rode from Gunlock to Enterprise and back, on the back roads and trails, mostly at cruising speeds but about 25% were at higher speeds. On my Polaris 325 Magnum, with a round trip of 73 miles I still had fuel when we got back to the trucks, even pulling another quad that died for about 5 miles. The others filled up in Enterprise but I had extra fuel in the Flat-Pak tank Old Syko is talking about so I purposely tried to run out to see what my mileage would be. Normally when riding mine always ran into reserve first and the others hadn't even got into reserve before we got back at the end of the ride. I would think that with one extra tank of fuel you should have no problems riding up to 150 miles, ok, maybe up to 140 miles, just to be safe. but if you don't ride all out full throttle all day you should get better mileage and have no problem. My Magnum has a 3.2 gallon tank. I think your Grizzley has about a 6 gallon tank. My uncle has a Yamaha 600, I think it is a Grizzley, and his has a 6 gallon tank and gets the same mileage I get.
As far as the water for summer time rides, I have taken many summer rides and found that the water bottles I carry on the bike are plenty. I usually carry 3 or 4 liter size bottles and they tend to last me for about 5 hours. If I was planning on taking a ful day ride I would just take a couple gallon jugs of water and not worry about it. Fill them half way and freeze them, then before you head out fill them with water and they will stay cold most the day.
Hope this helps,
Micheal