I can't speak for the area around Cabo San Lucas in July. I did some dives there years ago in December and managed in a 5 mil wetsuit with a hood. I got a bit chilled during those dives, but it was December. Cabo San Lucas's water temperatures are impacted by the Pacific Ocean.
Water temps in the actual Gulf of California warm considerably during the summer months. We generally expect water temps in the San Carlos area on the Sonora side of the gulf to be in the low to mid-80's in July. I usually wear a 3 mil, but others I dive with simply wear a skin or do the board shorts/long-sleeved t-shirt thing. If you're diving in the Midriff Islands, expect temps to be around ten degrees colder than the areas north and south of the Midriffs. The underwater canyons throughout the Midriffs are very deep and hold lots of cold water. Tidal and wind generated upwelling bring the cold water to the surface.
July is not a great visibility month in the Gulf of California. It's a good month for algae blooms and plankton. This is why locations like La Paz and Bahia de los Angeles have whale sharks begin showing-up in July.
How rough the water is in July really depends upon what's happening with the weather. Hurricanes have moved through the gulf in mid-July. Last weekend, we were seeing a the end of a swell event in the San Carlos (Sonora) area. The wind generated event was the result of a large storm that passed over southern Baja earlier in the week.
If it's a LOB trip out of Cabo San Lucas, they probably take you from Cabo up to the La Paz area. There are lots of really good dive sites along that route.
You might check whether or not the Los Islotes Sea Lion Rookery will be open during the dates of the trip. The government does a seasonal closure at Los Islotes during pupping and breeding season. It's not one of the largest rookeries in the gulf, but it's undoubtedly one of the gulf's most visited rookeries due to the ease of getting to it. Personally, I wouldn't schedule diving in the La Paz area without being able to dive at Los Islotes.
Opinions will vary, but I think the best months to dive in the Gulf of California are September and October. The water is usually warm and chances for really good visibility are better.
-AZTinman
