I have spent the last 5 years in the Sea of Cortez. Most people make the trip south at the end of hurricane season, which is late October to early November. I have made the trip down twice and back up three times. The cruising down there is amazing! Empty anchorages, warm water, great fishing and people. November and December are nice, but it does start to cool in late December. Things stay cool until about March/April, then they warm rapidly. Many people leave the Sea in May due to insurance requirements, but I leave my boat in La Paz year-round.
July is doable, but very warm. I just did 10 days this last July. August and September are super hot, and in the heart of hurricane season. Feel free to PM me if you have specific questions.
"There is simply nothing more worth while than messing around in boats."