If you have a cellphone with a camera, give it a try, taking digital pictures costs little more than time.
If you don't have one, and don't need one, I wouldn't suggest that one that will work well will fit into your budget, unless you come across an incredible deal.
Off camera flash is due to the reflections from a flash that is on the camera being close to the focal plane and basically being right at the lens itself.
Macro is due to the ability to focus close to the camera... You aren't necessarily going to be able to take a good picture of a fish if you are going to need to keep your camera 5 feet from the fish
A DSLR will allow you to manually focus, so you can set your camera up and wait for your fish to cone into where you have focused rather than trying to chase around the fish.
The more light will allow a smaller aperture to be used, allowing a greater depth of field, and hence more will be in focus.
Ultimately budget is likely to be the biggest factor at play here. You can hone your skills and work with what you currently have and then see if that meets your needs.
But look for guides online about photographing exactly how you want to. I don't know if he ever posted any guides here but Aquamojo posted some incredible pictures here a good few years ago, and continues to post pictures currently online.
I've also been pretty impressed by the work of Krish Weinholf, Karen Randall, Gary Lange and Chris Lukhaup.. but there are plenty of folks around the internet these days who take great pictures and give plenty of information as to how they do them.
You can always increase the chances of getting a good picture by taking many more, you will be more likely to get one that meets your needs that way.
You can increase the ratio of good to bad pictures by practicing, researching techniques and maybe having better equipment.
Also don't underestimate the power of using editing software to crop and clean up photographs after they have been taken.. you don't need to buy Photoshop, you can use something like GIMP for free.
I also think that Monster Fish Keepers has a dedicated section entirely for aquatic photography.
Hope that helps.