Hey, I have had a lot of experience in this - and there is hope for you! TBI Injection is fine for the reliability that you're after, and a complete kit with in-tank fuel pump, high pressure lines, filter, and the TBI will run you around $1500 and take a fairly skilled back yard mechanic 8-10 hours to install. Electrical work is also required.
Port Injection aftermarket kits for marine apps don't really exist - you'd have to retrofit from a totalled boat, including HEI ignition and the manifold. It would be a major hassle...
The good news is this: I've been through it twice on my last two PS190 boats. The simple fact is that marine carbs aren't built to last 15 years. That doesn't mean, however, that they aren't any good. If you want an easy, fast, painless solution, I'd go down to the local NAPA store in your town, and have them order you a brand new HOLLEY Marine carb for your boat (4000 series, probably). They retail at MC dealerships for $730, but go for $509 at NAPA!
Trust me on this one - I've done it twice, and my boats start on the frist turn of the crank, and run like a fuel injected boat. Most people can't believe how well my boats run...