I almost "invested" in an MSD unit for my 3.4L, but decided against it. Everybody told me exactly what John just told you: the ignition systems are pretty stout. MSD is usually just wasted money.

As for more torque/hp for pulling the boat, I personally don't know of too many mods you can do to boost the power w/o dropping a bunch of cash on a turbo or supercharger. I think a lot of guys report some success with intake modifications, though. Search around and you'll find TONS of info here.
And it wouldn't hurt to give your whole intake and ignition system a good cleaning (i.e. clean the mass airflow sensor, replace the plugs, check plug wires, clean the throttle body, etc.). A lot of times, those little things can make the truck run like crap and you won't even know it. <img src="/forums/images/graemlins/kewl.gif" alt="" />

[EDIT: Here's how to clean that throttle body]