SLR is not an option for your budget.. Just the housing + Port is minimum of $1600 (Ikelite). Anyway I wouldn't recommend starting out with a dSLR.
Micro 4/3 is an excellent option, but bear in mind that it's not significantly cheaper than dSLR. You still need a housing + port which is at least $1200-$1300. Even if you stretch that far, you won't have anything left for a strobe. Without overstating it - what makes awesome photos so awesome, is the strobe...
You might be able to find a second hand bundle with camera, lens, housing and port but I can't imagine that would be under $1500.
I would recommend getting a high-end compact such as the G16, which you can find bundled with the Fantasea housing. That would fit your budget and can provide excellent results. You can also grow and evolve with the system by adding strobes, wet lenses, video lights etc. and achieve very good results.
I highly recommend starting out with the YS-01 strobe, but if you don't have the budget for that, try to get the most out of the internal flash until you are ready for a strobe.