Since its creation Lake Powell has seen significantly varying water levels. It is fed primarily by the Colorado river which continues below the Glen Canyon dam. Inflow is subject to seasonal variations in precipitation and outflow is determined by demand for hydroelectric power and the needs of communities which rely on the Colorado for drinking and irrigation water. Because of this complicated system of water management Lake Powell's water level is very high some years and very low others.
What this all means for a houseboat vacation is very simple: some years there will be more side canyons accessible by boat and others you may find more beaches available. Lake Powell is so big that there will always be plenty of space for everyone. In many ways the fluctuating lake level means that your experience will always be different.
In mid- to late spring when snow in the Rocky Mountains melts, the Colorado swells, leading to the peak rises in Lake Powell's level. By summer and early fall when temperatures are perfect you can count on ideal conditions for a houseboat vacation.
Lake level chart above is courtesy of water-data.com.