Alex, you are correct, a separate strobe helps tremendously. Nice shots diveborg! One of the challenges with onboard flash is that you can get a lot of backscatter. Backscatter is the lighting up if particulate matter in the water column that can cause an image to look snowy. Diveborg's shots are macro, or close up shots, where there isn't a lot of water between the lens and the subject, so backscatter is really minimized in these images. So if you intend to shoot only macro, yes, the onboard flash may serve you for a time, but shooting anything at an arms length away or further will be a problem. Also, a separate strobe will also allow you to light your subject more creatively as you can position it different ways to shift your composition.
In the end, it's all about your budget. At one point you could buy a housing for disposable film cameras that ran about $60, or you can go all the way to housed SLR with dual strobes, multiple lens ports, and all the trimmings for $6-7000. And there are lots of points in between!