I know this is quite an old thread now, but thought i'd add my bit.
I've just done my advanced powerboat, 2 day exam prep and then Certificate of Competence with Stormforce in Southampton. Cannot recommend them enough, very professional, very well set up, excellent instructors using quality kit.
I could have done my exam in the Milford Haven Waterway, but having done it in Southampton I would thoroughly advise doing this on a busy seaport as it is going to teach you so much more. I reckon if I can operate safely in Southampton then I can do it properly anywhere, real baptism of fire but so worth it!