I don't weigh in her much anymore, but I'll toss you a few suggestions:
1. Trouthappy already mentioned that there are people guiding full time, out of Fountains and other luxury boats. Unless you can compete at the very top end, find a niche. Become known for it, and don't apologize. You can't be all things, and you'll kill yourself trying. Failure to focus dooms more small businesses than practically anything, second to being under-capitalized.
There are a lot of people who have figured out how much owning a boat actually costs on a per-trip basis, especially when they only make 5-6 trips a year. They decide it would be a LOT cheaper to hire a guide 5-6 times, and they sell their boat... but then they don't follow through with the guided trips. If I were a young guy looking to guide part-time, I would have to consider marketing to that niche. You'll get more-experienced fishermen, and if you do your part you'll develop a group of regulars.
2. If you don't know tax law, learn it or find someone who does. A part-time business can provide you with some write-offs. But if you don't run it like a for-profit business (the way the IRS defines it) you'll get yourself in trouble. Dot your I's and cross your T's and keep perfect records. When they assess back taxes, they throw in interest, and penalties, and interest on the penalties, and....
3. I always struggled to convince my employees that the busy times aren't what make or break you. It's making yourself work hard in the slow times to prepare for the busy times. Keep your boat in top shape. Keep it looking good. Make yourself a maintenance list, and follow it religiously. Let yourself start slacking, and you're lost.