Hi Hermit Crab,
This was exactly my dilemma recently. I have been taking topside photos for years and concentrate mainly on airshows and motorport events. I have a Nikon D300s that is getting a bit dated now but have a few excellent telephoto lens's. I looked at housings available for the D300 but felt that I would be making a significant investment for a camera that is a bit old now.
I have been taking photos underwater for two years with a Nikon AW 130 compact, no housing, no strobes, just point and shoot but with a great macro setting.
I have been very pleased with some of my results and just felt that I wanted to go a step further and be able to shoot in manual as I do topside. I spent months researching my options. The cost of replacing the D300, plus housing, lens's, strobes etc was just a bot to far out of budget.
So, I decided to keep the Nikon D300 for topside and went for the Olympus OMD E 1 MkII. As a 4/3 not quite as good as a fullI DSLR but significantly cheaper. I went for a Nauticam housing and a pair of Inon Z240 strobes.
I only got all this last Wednesday. I have managed 5 dives last week (3 on a Underwater Photo Workshop) and each dive was a big improvement on the previous. My first dive I felt like a new diver. Buoyancy all over the place and not very comfortable with all the gear. My second dive I removed my normal trim weights from high up in my BCD and that made a huge difference to balance.
Photos got better and better, I have a macro lens but mainly concentrated on close up wide angle. To be honest I have a long way to go to produce photos that are of a high standard but I am now confident that I will get there. Handling the camera underwater is now ok.
I feel that I can justify the investment as I love taking underwater photos but was just so limited with what I could achieve with a compact.
Whatever you decide, have fun and enjoy.