The Wave Destroyer is the perfect boat for what you are describing. I've been rowing them since the prototype stage, and they are solid. I've run it loaded with myself and gear on the Grand Canyon (it carries a load a bit better than the similarly sized Ocelot, in my experience). I've also used it for the North Fork Payette top to bottom. Many of the other boats mentioned, like Maravia, work great for many things, but the WD has a class V performance edge on even the updated Maravia design. (Though, as mentioned above, Maravia's construction quality is nothing to scoff at. They build a good boat.)
Even though I'm an AIRE guy (by preference, not because I'm being paid off), I think the Legend is also a good boat. When comparing the Legend to the WD, keep in mind that the Legend has fatter tubes. The 12.5' Legend is almost equivalent to the 14' WD in terms of load capacity and other associated size considerations in how it runs.
So, if you can get a WD, that is my first recommendation. It will be a forgiving design for your dad to row, and it will perform excellently when you want to do a harder river.