There is no easy answer to your question but I will try to give you some of the things I have learned since I started fishing Erie in an brand new that year 1955 16 ft Thompson.

You have already made a true statement in that the wind direction has a lot to do with the waves generated. There are several factors that go into that and of course the first is the effort or speed of the wind, the second is the fetch or distance is has to use that effort to create waves. On Erie the most fetch is generally out of the Northeast and thus the largest waves but there is another factor often over looked but important this time of the year and that is the temperature of the air doing the work. The colder the air the denser it is and the more push it will exert at the same speed as warmer less dense air. You mentioned a 20 mile south wind. First thing that comes to mind is a high pressure area pushing very warm air north. The would not cause me a lot of concern within 5 miles of shore however the ability to control your boats drift speed in that kind of wind might be.

Generally speaking any wind direction with a speed of under 15 mph in temps of 65 to 70 will give you not more than 1 to 3 foot waves but heres another kicker, when you see that forecast of 1 to 3 , 2 to 4, or 3 to 5, I always add the two numbers together and that is the size of the so called rogue wave you could and probably will encounter.

An 18 ft boat will handle a lot of rough water what usually gets a boat into trouble is speed, either too much of it causing water over the bow or too little when the power is needed to maintian steerage

As I mentioned earlier wave size and wind speed affect the quality of the fishing long before the ability to get the boat out there and back.

I hope this has helped a little. but nothing will replace your own experience and the mental notes you'll make to yourself on some days.