OK Let me chime in here as I live at Lake Powell.
July is warm (we call it warm, those from Seattle call it hot) and can get windy in the afternoon. You won't find a better lake for your Gathering. It's 180 miles long and has more shoreline than the distance from San Diego to Seattle. 500' sandstone cliffs everywhere, sandy beaches most canyons.
Buoys mark the center channels and their numbers denote how far up lake you are from the dam.
Fishing is excellent in the lake. Bass (both small mouth and large mouth) catfish by the hundreds at night, crappie, bluegill and no limit stripped bass fishing from 2 to 8 pounds average. Some people catch 100 per day easily ( check out http://www.wayneswords.com
for the fishing report every week).
Water temp at the top surface will be high 70s or low 80s Ten feet down it will be in the low 60s and 50s.
Not to be missed is a tour of Antelope Canyon just east of Page, Lots of tour companies in downtown Page for this. Also absolutely not to be missed is the Colorado River tour below the dam to Lees Ferry. Big blue inflatable 25' boats hold 20 people each with NO white water on the trip but you'll be in the canyon with 900' sandstone cliffs, wildlife, 1000 yr old Indian markings, etc. This you have to do. It's done by Colorado River Discovery, a company in Page. It's a half day trip.
There is a fishing dock open all night at the east docks.
All boats brought in will have to undergo an inspection for quagga mussels before launch. It's done right at the launch ramp.
Sailing can be fun and challenging on the lake with the wind currents changing at every canyon. I've done it and it's fun.
Distances are big out on the lake. Going up lake can easily eat up 30 or 40 miles and you haven't gone that far on the lake. Watch your fuel. Always start out with a full tank. The next fuel dock up lake is about 40 plus miles up lake near Rainbow Bridge (if the cut is open, if not figure 65 or more miles the long way). Rainbow may be closed even next year due to flooding this year and washing out the trail and docks.
When on the lake, if a thunderstorm pops up and the wind picks up, head for a sheltered cove out of the wind and sit it out. I've seen 6' plus wind waves on this lake. It'll die down in a couple of hours usually. ALWAYS HAVE LIFE JACKETS AVAILABLE AND ALWAYS WORN BY THOSE UNDER 12.
The Sheriff does patrol and sometimes waits at the dock with breathalyzers. Always have one boat driver sober. I only say this because this is a big summer party lake with the skiers and houseboats and the Sheriff knows how much beer is used on the lake. I've personally seen a house boat launch with 100 cases of beer on board for a 3 day weekend!
Cell phone service is hit or miss on the lake but Marine radio works most every where for the Park
This is a great place for your Gathering ( that's why I live here!) You guys will have a fine time.