As you already know the issue is you are loosing color as you are loosing light. The only solution for this is to use a strobe (external flash). They can be rather expensive and involved, but without one almost every one of your photos won't come out. The difference is going to be shocking to you! Once you try one, you will understand completely.

You can try to use one of the photo editing programs you mentioned, unfortunately none of them are going to be able to work magic. You may be able to brighten up the images and add a bit of color, but the results will be minimally better.

Unfortunately I have been in this situation before, taking hundreds of photos while away on vacation, only to find out that the photos are basically useless(yes, film cameras not digital back then).

I am still unable to find a satisfactory underwater camera setup in my budget, but I have been fortunate enough to have friends that do. I also rented an U/W camera setup before, it was actually pretty reasonable and worked out very nicely. Unless you are diving on a regular basis a rental may financially be the best bet.

I hope I didn't get too far off the subject.