Nephrectomy doesn't contraindicate diving. I gave my spare to a friend a few years back, and the local DAN doc here in town said it wasn't an issue, once it'd healed. I gave it 6 months just 'cos of the pulling I was getting the first few months. My surgeon DID make me promise to give up skydiving, due to the possibility of injury to the remaining kidney.
Never had any trouble with it on any dive, although I've finally given in and started bald-faced lying on all of the med releases and saying "no problems". Got tired of some idiot divemaster thinking he knew better than my DAN doc (who'd signed me off). Here three years later, it's just a thin line, and I don't miss it at all.
Amusingly, one fool divemaster thought that there was a gas pocket internally where the kidney had been, and didn't believe me when I told him it'd likely been filled in with fat long since, even if the surgeon *had* left a small amount of void in there. The guts just kinda flow around to fill the empty space, and any air pocket would have been absorbed by the tissues long since....
As to the original poster's question about kidney infections due to diving, I'd think it would only be an issue if you're diving in an area contaminated by sewage or other biohazards. I'd hope the dive operator kept tabs on any local bio problems from effluent discharge. I haven't -heard- of anyone getting a serious infection from diving (other than otitis).