we were in havasu last spring during spring break. as long as you stay out of the channel it's not bad. weather was warm. lots of people around but we had lunch at one of the more popular mexican joints and no wait to get in. lake looked pretty deserted for tubing/surfing/boarding.
if you time your week right, they do races down at the Bluewater south of the dam. short drive down the canyon to put in on the lower colorado for a day.

not sure on mead's water temp at that time. temple bar is pretty inexpensive to stay at with everything you need right there and day trips to vegas/hoover dam/grand canyon.