The diver should be washed down with chlorine, or chlorine bleach once he is fully suitted up. You will have to contain the waste water because it is considered a hazardous material in most states. The diver also is supposed to be fully contained in a dry suit with double exhaust valves. This means that you have to have a diving helmet that will mate to the dry suit. Neck and wrist seals just don't cut it. There are a lot of divers that will argue that their full face mask and a recreational dry suit is all you need, that is wrong because you are dealing with drinking water, not some marina that people pee in.
Safety wise, it is a minimum three man job, no shortcuts. It is very easy to get lost, entangled or have any other issue a diver can face cause a fatality. SCUBA is not a safe option. There is no safe air space in a drinking water tank, the chlorine vapor concentration inside is usually high enough to cause some serious respratory damage. Chlorine is used for disinfecting water because it distroys living things, like virus, bacteria and live tissue. You must be prepared to retreave an unconsious diver out of a poisonious environment, usually up a ladder while working off a platform a couple of hundred feet above the ground. A standby diver might be a viable option for safety, but he must be equipped with a dry suit and helmet or you will be paying to have the tank disinfected and tossed off the job site. Also, a standby diver requires a second tender. One diver = one tender, no exceptions.
Other than that it is all logistics, how do you get to the hatch, where do you put the compressor, where do you put the dive station, etc. If is a tall tank you might need four guys, a tender and dive supervisor at the hatch, diver in the tank and someone to the compressor and or run the radio and air rack.
The worst part is selling a three to four man team to the water company because they have probably hired scuba divers in the past who did not follow any OSHA requirements at a cut rate, and the water company people probably don't know d**k about what it takes to run an underwater operation.
