Re: 18 ft walkaround with 4.3L OMC cobra, ok to go offshore fishing?
I'm going to take a bit of a different track here and not talk about the equipment you take, because I believe that if you are experienced enough to do what you want to do you shouldn't need the equipment (spare parts aside - always good to have them and to know how to replace them). Yes the electronic navigation do-dads are nice, but if you can't find your way home with just a compass I'd be concerned. - Some of your questions also give me concern. Wanting to get to an oil rig and then asking "How can I tell where the oil rig is at the closest to me in Galveston" is concerning.
It is the experience that takes you safely out and back, not the boat. Back in the mid 1950's my parents would go a couple of miles offshore in a 12' plywood outboard runabout with a 15hp Evenrude on it and were perfectly comfortable. I've been commercial scalloping 100 miles offshore in a dead fog and had a guy idle up in a Boston Whaler Outrage and call out "which way is Atlantic City?" - him I'm not too sure about.
Once you learn more of what you are doing you won't need to ask the question 'can I do this'.
With experience you will understand how quickly water conditions change with changes in the weather. You should understand basic navigation and not rely completely on the electronics as if you are staring at a video game monitor. Understand direction, steering on a heading, currents, wind changes a bit, understand your boat and how it handles in different conditions and how it handles as the weather decays. It's one thing to be a 1/4 mile out and realize the weather is turning nasty and 'oh crap I'm just figureing out what my boat can handle' and being 20 miles out and have to figure the same thing out. I'm not saying saying you need to be an expert in any one area, but you should know enough to understand in advance when you may be heading for a problem situation.