PS--- The fact that Desolation might be crowded does not mean it's not worth going, particularly if one's only window for going there is July-August. The place is still beautiful and there are still all kinds of cool things to see and do.
The difference will be in the challenge of finding spots to anchor and the need to put up with more noise, dinghies zooming around, and sometimes annoying, rude, or stupid behavior on the part of other boaters.
While the off-season, particularly September, still sees boats up there in some spots, the boaters themselves are very different in our observation than the July-August crowd. The September/October boaters tend to be retirees, some of them returning from longer cruises farther north in BC or SE Alaska, and thus much more experienced (yes, this is a generality). Because of this they tend to have a lot more respect not just for their fellow boaters bur for the place itself.
In our experience and observation, this same thing holds true in the San Juan and Gulf Islands, particularly the Gulfs.