Im from northern california and have spring breaked in lake havasu in 2004, 2005, and 2006. Also went to havasu for memorial day weekend in 2006 as well.
First off, check Arizona state and university website and figure out when THEIR spring break is. This is the BEST week to go. In 2004 i was unemployed and not going to school so i went that week and had an absolute blast.
2005 and 2006 were consequently weak, because my community colleges spring break was the week after, so everyone i met was either from montana, or arizona, because my colleges break wasnt even the same as much of southern california.
You say you will have 6 people going? If one of you guys has access to a van, i HIGHLY reccomend you drive, you will save a lot of money compared to flying. If you do choose to fly you can go with either Las Vegas NV or Phoenix, AZ ... both are within 3 hours of lake havasu.
Boat rentals are plentiful, expect to spend $300 per day or so, i reccomend getting a pontoon. 2006 i just bought an aluminum fishing boat for $500 and towed it out there, my gas mileage suffered and i had the crappiest boat on the lake, but i didnt have to bum rides to copper canyon from other boaters. If you have more GIRLS in your group than guys and they are reasonable attractive, you can normally just hang out on the channel for a while and ask boaters if they are headed to the canyon or sandbar, and a lot of times they will let you on. In 04 and 05 i normally offered people beer, and since i was by my self, i could get out to the canyon before the day was over.
London bridge hotel is the hotspot, because the bridge and channel are the central hub for everything. However there are at least 5 other hotels within a mile radius of the london bridge hotel which are many times cheaper.
If you guys choose to camp, you have two options. You can either camp at the crazy horse which i think is over $20 per night during "holiday periods" ... otherwise you can go to BLM land which is free. I have always camped north of the airport without any problems, but if you feel more adventerous about 1 mile south of the london bridge is also BLM between AZ95 and the lake, many people launch boats there so its a highly trafficed area so you may be more suseptible to vandalism, ive never camped there myself but am seriously thinking about it for 2008. One note about camping is dont drink too heartily and park your car right behind your tent on the east side so you will have a little bit more shade from the morning desert sun.