There is always tons of spots to ride, you just might have to drive a while to find one and/or wait out any potential wind and such. There is more shore line on Powell than the west coast of the US. the main channel is about 180 miles long alone.
speaking of wind, (not trying to scare you, thats the last thing i want to do. just giving you some info/ advice from a Powell resident) July is the hottest time of year and is also during the "monsoon season." Tail winds from tropical winds can reach up as far as Powell and kick up some real sudden storms. If you have seen May Day, then you will have an idea. The storms are usually short and are not daily or anything. If you find yourself caught in one it is best to wait it out, preferably away from the wind. Keep an eye on the sky, if you see any sign of storm clouds, stay close to "base" and you will be fine. (wherever you have designated base as) (wherever that be for you) Basically it is the ignorant people who think a lake cant get that dangerous that have problems... and now you are surely aware. You may not even experience anything crazy w/in a week's period.
If you want to be close to civilization and have more amenities available to you then i would go to the arizona side of Powell (5% of powell is in arizona) and use Wahweap and/or antelope marinas for launching. Wahweap Bay, warm creek bay, and padre bay are all close and are the biggest bays of the lake. There are also other natural attractions near Page Arizona (only city near powell, my home town). Horseshoe bend, (see wakeboard graphic below)
antelope/ slot canyon (check these images
http://www.google.com/images?q=antelope+canyon&sourceid=navclient-ff&ie=UTF-8&rlz=1B2GGIC_en__214__214)
If you need to rent anything then i would do it NOW! check
http://www.lakepowell.com and/or
http://www.antelopepointlakepowell.com/index.cfm for details. also you can do a search on any desired services in Page AZ and/or Big water UT (another very small town near powell)
any other questions you have? i and others will be sure to help