Water Capacity - You should be able to go easily with 4 liters per day for drinking in the hotter sections. Bring more for cooking. Cook meals near water sources, then fill water bottles and move on.
I met several thru hikers a week ago, and water had not been an issue.
Water sources -
Lost Valley Spring
Verified a week ago:
Live Oak Spring
All streams in the San Jac Wilderness
Long Sleeve Shirt, light color makes a big difference. If you don't sun burn, then a t-shirt is fine. Bring a wind shirt.
Last weekend the low's below Mount San Jacinto (below 9,000') were close to 50F. You will be fine with your bag. Ditch the other insulating stuff.
Not much in the way of insects in the San Jacintos, so I would skip a tent. If no rain is forcast, just bring a poncho/tarp if you have one just in case. Temps in the days are probably going to be above 80F, so I would go as light as possible, other than water, so you can make easy time between water sources. This hike is mostly uphill, until you descend down into the San Gorgonio Pass, so with the temps and spotty water, Ultralight is the name of the game; definitely a 5lb base or less if possible. There is not much you really need, except food and water. You might look at the thread I posted on the Skyline Trail loop, I would use the same equipment, except use my BPL UL 60 quilt. I brought the Nanatek, just because I wanted to try it out.
If you need any more help, just ask and I will hopefully be able to assist.