I don't know. I think you are going to have a very tough time finding what you are looking for. First of all the equipment you are looking for is very specific and very high end and very complicated. So I doubt that anyone has them for rent. The best I could find was a D2X or a D300. You can rent them from Cathy Church in Grand Cayman, Stephen Frink in Florida, or Backscatter by mail order. Perhaps some of the specialty live-aboards that do photo cruises have high end gear to borrow. But A D3/x? In any case its expensive, upwards of $350/day and over $1800/wk even for these older systems. That includes shipping time.
But I think the better question is what do you hope to accomplish with the system if you had it for a day? These are very tricky tools to use with a lot of task loading. To get your money's worth on these cameras you really need to know what you are doing and have some practice with it beforehand. I doubt that you will achieve what you are looking for by having such a high end camera for a day unless you are really familar with that camera, underwater photography, and the housing/strobe system.
This is why Cathy Church's camera rental comes with a dive guide included @ $60/hr. I think this is wise. In any event I can highly recommend her school in Grand Cayman. You can shore dive right off her dock and her instructors are top notch. She helped me figure out my point and shoot digital camera and Ikelite strobe years and years ago (before dSLRs existed). She spent a couple hours figuring out how to make it sync and I had a 1 hr dive with her instructor. She only charged me for the dive. I hadn't bought the gear from her but she was a very nice lady.
I think you really have 3 options: Take a structured course, buy your own high end system and take the time to learn it, get a point and shoot renrtal and practice. You can buy a point and shoot rental with strobe for what you'll pay in rent on the system you are seeking.