I have been asked to do a shoot underwater (in a swimming pool). Wondering if anyone can tell me what I would need to rent or buy. I see an underwater housing online that costs as much as the camera. I have a couple of months to prep this, so no emergency.

Alex,
If it's a one off shoot, then i think your best bet might be to rent one. Failing that, they the Ewa-marine like this should fit the bill.
Ewa-Marine VXA Underwater Housing EM VXA B&H Photo Video
Regards,
Bryce

Thanks. I hadn't seen that one, but one similar. I just need to find where to rent one.