Hi john
Let's go back to basics when shooting underwater.
Take as much water out of the equasion as you can between you and your subject. It matters not how many MEGAPIXELS one has if there's lots of Water based debris between you and your subject, image clarity will be poor forcing you to spend lots of time in photoshop attempting to correct it.
Move closer to your subject rather than zooming in. Most modern lenses such as 10-17 or 12-24 allows one to get close enough to force out that H2O between you and your subject. You might find that housing the 17-70 will work great for you with what ever it is your looking for in your images however you also run the risk of being "lazy" and zooming in.
I find that 12-24 is perfection for me. I have used 18-70mm underwater but I generally used it when jumping into the unknown for the first time and my focus is on items of equipment a diver is using.
All you need to do is give it try , grab an extension ring and give it a go. If you have to buy a new dome. I just don't think it's worth spending the extra on the dome to potentially be unhappy with your results due to turbid water conditions.
Grab a 12-24 if you don't like the fisheye , they are an aquired taste.
I just use a 40mm extension on my dome to switch from 10-17 to 12-24mm
Again 105 or 60mm. How close can you get ?
Hope this helps
Craig