I'm well aware that working, remotely controlled Technic boats based on compact LEGO hulls have already been confirmed as a working concept, but I just couldn't help giving it a little try myself, in the catamaran layout. Nothing extremely developed or fancy ― the intention was to just have some fun testing it at the nearby lake.
It turns out that this design works rather well, and the boat was both swift when running under full throttle, and maneuvrable. Instead of having a drive & rudder motor configuration, it actually uses two parallel propellers, each connected to its independently controlled Power Functions L motor*. Turning is, just like a tank, done by turning the propellers at different speeds, or in the extreme case, turning them in the opposite directions (i.e. a differential drive). They are controlled by an IR V2 receiver and powered by a standard battery pack.
The boat was pretty stable and did not rock too dangerously even when maneuvering rapidly. Design allows the battery pack to be mounted at several points longitudinally (along a Technic liftarm), so that the overall balance of the boat could be finely adjusted in the center. It turns out I was lucky, and the initial configuration from the workbench was already working well.
What wasn't working so well were the propellers which had the tendency to collect every bit of weed floating around the lake and get stuck, and the IR controls which struggled with the intense sunlight, reducing the control range to just barely a meter or two. Of course, in theory an NXT smart brick could be used instead, which offers the extended range of Bluetooth communication, but I simply didn't want to risk ruining it in the water if anything went wrong. Fortunately, it didn't anyway, and the boat actually easily handled some extra weight of a GoPro camera.
In any case, it is not too difficult to build (just make sure it's sturdy and simple), and the hulls can be found on BrickLink for reasonable prices, so give it a try if you feel like a LEGO Captain!
* Thanks to Sariel for the transmission idea