The best software for image editing is Adobe Photoshop. The only problem with it is the price, which is normally around $350. If this is too much, then I would recommend Photoshop Elements, which does much of what you need from the full photoshop product, but for under $100. The alternative would be something like JASC Paint Shop Pro, which is about the same price as photoshop elements, but IMHO much more versatile.
If you still have the negatives, you can also try taking them to a professional photography shop and have them develop you some prints from them. If you explain that they are underwater photos they can compensate for the colour loss in the printing process.
Hope this helps.