In Tomasis book, this is on page 170
In Martins book, this is hike ....
Don't confuse Bathtub tank in Adobe Canyon with Bathtub Water in Big Casa Blanca Canyon. They are about 1.5 miles apart and in different canyons. Also note that Tomasi says nothing (that I have found so far) about visiting Bathtub Water, but he does say a lot about illegal traffic in this area.
You drive down highway 83, through Sonoita, then southwest on 82. As you head towards Patagonia on 82, the milepost numbers decrease. You will pass the Adobe Canyon Road between 27 and 28. You want to watch for MP 25, and at 24.5 (before you get to MP 24) you will turn north/west onto FSR 152 (Casa Blanca Canyon Road.
Little and Big Casa Blanca canyons split about 0.7 miles up the road, so it is OK to simply talk about "Casa Blanca Canyon" at this point. You can drive FSR 152 for 3.2 miles. It ends at a gate and private property, but about 150 yards before this happens you can turn right onto FSR-4105. You then drive FSR-4105 for 1.6 miles to a parking area at a 3 way intersection just east of peak 4913.
FSR-4107 branches off here, heading northeast. The recommendation is to park and hike from this intersection. An experienced driver can doubtless drive farther on either road. As Tomasi says, which would you prefer? A nice hike or a miserable drive?
Tomasi describes 3 hikes from this location.
Tom's hiking pages / tom@mmto.org