I started out with an SP-350 as well. I still have two in my drawer (used to take them as topside cameras when I switched to DSLR). The SP's could take nice pictures sometimes, but had too many limitations: Slow focus, slow writing, too much hunting on macro shots, too close focus on super macro making it impossible to light the subject, expensive Oly/Fuji cards that were slower than cheap competing cards. Don't waste your time with the 30D. Nice camera but hopelessly outdated. Will take all the expense of buying related items, but not give you superior results.
Three choices:
1. Stay with compact and get a Sony RX100II and one of the cheaper housings or a Canon S120 with Canon or other reasonable housing. Both are 10 times faster than the SP and I've seen total noobs take ridiculously good pictures with the Canon S models. If you like macro, you'll need a macro lens to make the Sony work. If you like TTL, I understand you won't be able to use the Canon in manual mode and get ttl flash.
2. Wait for the next generation Oly mirrorless to come out and jump in on the super sale on their last gen with Oly housing. Several of my friends have done this and are taking great pictures with a total expenditure similar to what you seek.
3. Look for a previous generation DSLR kit on Scubaboard or Wetpixel classifieds, or E-Bay. I saw a set-up like my current Sea & Sea housing/D300 go begging for under $1K with everything but strobes and arms and lenses. You won't do it for under $1200, though. The lenses cost too much. Count on $400-$800 per lens on any DSLR and you'll need at least one macro (60 or 100/105mm) and one wide (10/17 or 12 to 15mm fisheye) plus a macro port and a dome. Canon T3-T5 models or Nikon D70/D7100 or D300 would probably be best, but doubt you'll find a D7100 in your range.
If I were starting fresh, I'd probably go with a compact. With the new airline baggage fees and carry on restrictions, my wife and I have to combine our carry on and personal items to support my DSLR habit. I will...not...ever check my cameras and leave the reason for my trip to the hands of TSA and baggage mishandlers. Broken chargers and missing batteries and broken dive computers have taught me otherwise.
---------- Post added July 20th, 2015 at 08:05 AM ----------
It seems there are still a couple of the OMD EM-5 packages out there at $1299. If you're thinking about a detachable lens set-up, you can't go wrong with that.
Olympus OM-D E-M5 underwater housing bundle with camera, lens + zoom gear | Bluewater Photo & Video
