You Natal guys have a bunch of very good boats up there that suit surf launching and diving very well. I bought an Orion 15 cat many years ago and she has given excellent service. The low gunwales are a treat for getting on and off so you can jump a bunch of yellowtail schools in the Cape without getting hammered.
I am, of course, a little (OK, maybe more than a little
) biased, but I think that you will struggle to find a better dive boat. I have had her out in some really horrible weather many times (by accident before the advent of the great online forecasting tools we now have), and I figure as long as the motors are looked after and you carry enough fuel, you can get through anything. I have often had her out for tuna off Cape Point, and made a number of memorable trips back to the point, from as far as 33 miles from the point, which is another 7 miles from the launch site.
One day I was out diving with two mates and we came back up False Bay past Hell's gate in what I estimate as 50 knots of NW. We made no more than 7 knots and all 3 were right up front, we had to wear our masks & snorkels to see anything!
I probably fish off her now more than dive, although I am treating her to a long awaited trip to Ponta in June. But as a dive boat, I have not seen a boat I would rate the same. The 18 foot Butt cats are good as well, but then you have a much heavier boat.
There is also the mono vs cat debate, but in small skis I rate the cat above the mono. If I were crossing the ocean in a sailboat, then the mono is an altogether safer boat in a big blow, just not as comfortbale as a cat when it's calm