Hi Andrea,
I bought this camera (without YS 20 at first) without thinking too much about U/W photography. As you might know from my name, newbie, abc...etc.
Developing the films after taking snapshots in couple dives, I was totally disappointed. It did not work at all, most of the pictures were blurred by backscatters. (As expected by experts here)
Then, I got the YS 20 strobe for MX 5II too. I would say if it improved a lot regarding backscatters. You can even use one of your arm as strobe arm. It fires simultaneously with the internal flash. But you have to block the internal to avoid it from generating backscatters. The cliper has to be added/modified by the official dealer.
But as personal opinion, do not invest too much on MX5II. I saw a Japanese diver taking picture by P5 I think? of course with its housing and a red filter. It gave clear and light pics without flash. Then I decided go digital, coz I can watch what I got U/W. It is very important for a diver without too much knowledge about shutter and aperture settings.