In most of the villas the water heaters are inside the bathrooms so if you switch them off you can have enough water for a shower.
Now they have started to built villas with the water heater outside : esthetically it is better but makes it impossible to have a shower day time!

If you want to have a nice fresh shower, I believe that the best is:
1/install a cooler on your water tank (insulation will do little job but not a lot) but I think that when they install the cooler they also do some insulation on the tank. Check previous post where a person is offering his services http://www.qatarliving.com/answers/family-life/my-shower-help
2/ become member in Ritz/Intercom...and have a nice cold shower there :-)

Solution 1 expensive (I heard 6000QR to be checked) but still cheaper than solution 2 :-)

I am tempted to install a cooler too so please QL readers share your experience!