I have Columbia's omni- brand jacket and pants that I always carry in a pack or bag. The omni-brand is one of those Gortex knock-offs that can be had for 1/3 the coast. Jacket + pants cost about $75.00. They are quite packable (fold into their own pocket), wind resistant and provide some protection from cold when used over insulating layers. However, like Storl, I find that they are less than ideal in real downpour situations. Even though the tag claims full waterproof, they certainly don't appear to be. I suspect that it is poor DWR (durable water repellency applied to the outer surface) that lets the face material soak, cutting breathability, and letting you stew in your sweat. Just about everytime I wear them in a good rain I get soaked.
In cooler spring/fall weather, and when I know the rain is beating down, I opt for my true 3-ply gortex jacket/pip-pants (MEC-brand). These things always keep me bone dry, even when waves are washing over me in the back of the boat. They also weight about 3 times the omni-brand ones and don't conveniently pack into their own pockets. They provide a lot more insulative value when used as a shell though. When these things get wet, the water just beads and roles off.
As for those heavy duty yellow slickers - the old man in the sea can keep them. I sweat myself soaked in them after about 2 minutes.
While I haven't ever tried it, I suspect that Poncho's don't work on boats - too much wind and can interfer with your ability to find a handhold. I can see their use as a shelter though. Are their gortex ponchos? Seriously I'm in love with that stuff.