So they spend very little. They still provide an increase on the sales at the supermarkets, gas stations, clothing stores etc, etc, etc that if removed would impact the local economy. It's simple economics.
For the sake of argument let's say there are roughly 5k winter visitors each year in LHC. I am using 5k because is the number I have always heard, it could be more or less. Assuming most are couples that is 2500 couples in town. Even if those 2500 couples are spedning ONLY $100 per month in town, and that is a VERY conservative amount, thats $250,000 (100 X 2500) PER MONTH.
Now, take that and multiply it by the typical snowbird season of 4-5 months that's possibly as much as $1.25 million removed the LHC economy. And that does not even take into account the rent/lease amounts the snowbirds are providing to the homeowners whom they rent from, whether local or from CA, which is additional money that may be pumped back into the economy by those homeowners, quite possibly in the summer. Granted, summer visitors may very well spend more total dollars, but Havasu still needs the winter visitors AND the summer visitors to sustain the local economy.
Now to take your assumption and twist it a bit. Let's take one weekend at Havasu during the summer. A very conservative estimate of the number of boats in Havasu would be 1000 (but we'll use it). Another very convervative estimate would be the amount of gas that each boat will consume (averaged out would probably be 80 gallons per weekend, some use less and some more). Now at around 2.00 per gallon that would be $160 per boat (again, average). That's $160,000 in gas for a weekend. Then for the season, $3.8 million. The summer crowd has a more expendable cash flow then do the "snow birds" that are on a fixed, usually retired budget. Go into a restaraunt and ask the people who work there which clientel they would rather have.