I believe that all Olympus digitals have a pre-flash that doesn't synchronize with standard strobes. The C-5050 allows you to cancel the pre-flash. If the D-40 and C-750 don't have the pre-flash cancellation, you're stuck with using strobes designed specifically for digital cameras that have a pre-flash. That can run you hundreds of dollars. Your choices are Epoque (don't know prices), Sea & Sea YS-25DX ($300-500), Sea & Sea YS-90DX/Auto ($500-700), Ikelites ($$??) and upwards. I rarely see the digital compatible strobes sold on EBay.
When snorkeling, will you freedive down to the level of your subject, or take photos from above? Photos from above don't work very well, and if you look at the tips section of this board you can find some really good suggestions on composition and camera angles.
Happy and safe breathholds for you.