When we first started ocean boating I realized that there would be a day when we would be caught in conditions that were far less than plesant. If I didn't develop the skills to safely handle my boat in very bad conditions my family and I would be put at risk.
Like most I started out by reading. The links below are to some of the very best articles on Rough Water Seamanship I've been able to find.
Rough Water Seamanship Part I: Boat Handling
Rough Water Seamanship Part II: Boat Handling
Getting Caught in Thunder Storms - Rough Water Seamanship Part III :Boat Handling
I don't know how the rest of the world builds skills but my method was practice.
I started out in a little rough conditions. With time as I learned, I started going out in Small Craft Advisories (when I had a small craft).
Doing this built skills, plain and simple. Learn by doing. I learned how boats handle, I learned about confused seas, and about beam seas, and about how to make the best out of a bad situation.
Eventually I learned enough that I ventured out in Gale Warnings. By this time I had moved to a 34' twin engine Bayliner, and I wanted to have the skills to survive. Learning those skills was not easy, but it can be done.
Now, years later when we get caught in unplesant conditions I know what to do. I know how to handle a boat in rough seas to make it back to calm waters safely and as comfortable as possible.
Rough Water seamanship is a learned skill. We are not born with it. I would suggest that anybody that takes boats out in the ocean take the time to learn those skills, so that you will have them when you need them the most.