It shouldn't take much pondering on my Beaufort Scale post to realize that it's kind of simplistic. Most people who have been to the beach have seen waves coming in on a windless day, often very big waves. Almost everyone has seen mud puddles on gusty days and realized that there are no 20 foot waves forming in them. Straight use of the Beaufort Scale can be a good general guideline for rulings on waves, but leaves a lot up in the air. Why is that sailors were more comfortable leaving sight of land in the Mediterranean than in the Atlantic in the early days of sailing? Why are small boats acceptable for working on lakes and inland seas not suited for the open ocean?
When I do research for a game, I start with what I like to call the "picture book" level. You can get a lot of feel for a place and time by looking at pictures and diagrams of the tools and culture. Books like Edwin Tunis's Colonial Living and Frontier Living, Richard Humble's Ships, Sailors, and the Sea, and The Diagram Group's Weapons: An International Encyclopedia from 5000 BC to 2000 AD prove that a picture can be worth a thousand words or more. Many times, depending on the topic, this is as far as research needs to go for the purposes of the game. I consider the Beaufort Scale to illustrate the weather on this level of research...a simple summary that needs little or nothing done to convert it to game use.
But when the picture book level isn't enough, I move up to the "college intro" level of research. At this level, I end up reading books and websites aimed mostly at high schoolers and college freshmen or hobbyists. A lot of game rules fall into this level: the planet-building rules and sublight travel rules for Traveller, the plate-tectonics, meteorology, and oceanography in the 2nd edition AD&D World Builder's Guide, and the engineering in GURPS Vehicles. When I did the college intro research to add the missing detail from the Beaufort Scale, I was able to answer a lot of questions, but then found I had to do more research.
So what did I find? It (unsurprisingly) takes a little time for waves to develop. It also takes some distance over water for the wind to blow (called fetch), which is why lakes and enclosed seas are safer for smaller boats than the wide open ocean.
The 2002 edition of the American Practical Navigator (downloadable at http://msi.nga.mil/NGAPortal/MSI.portal?_nfpb=true&_pageLabel=msi_portal_page_62&pubCode=0002 for anyone interested) has a somewhat more game-able chart on page 453, which I have made even more game-able by rounding decimals off into whole numbers, and working the fetch lengths out into multiples of 12 miles (in deference to most game maps being at scales of 6 or 24 mile hexes). I also eliminated redundancy on the table by only keeping breakpoints where either the wave height or the wave period changed.
Here is a sample of what I have so far. To use these tables, find the subtable of the Beaufort Number currently in effect, then cross reference the fetch and the amount of time the wind has been blowing. In the middle of the ocean, fetch is basically unrestricted and you can just look at the time, but lakes and seas can be much more forgiving. For example, Lake Halli on the Wilderness Map of the Great Waste in X4 Master of the Desert Nomads is about 60 miles across on its long (NW-SE) axis and generally about 36 miles on the N-S axis. If the wind is blowing at Beaufort #5 from the north, then in 3 hours, Lake Halli will have 4 foot waves; three more hours increases wave height to 5 feet, and in two more hours the waves will be fully developed at 6 feet. Due to the limited fetch, the waves can't get any higher. If the wind shifts around until it comes from the northwest, the increased fetch will allow the waves to grow to 7 feet. Even under winds of Beaufort #8, waves won't exceed 17 feet on Lake Halli. Wave height (in feet) and period (in seconds per cycle) are limited by the smallest of Beaufort Number, fetch (in miles), and time (in hours). I have tables for Beaufort Numbers up to 11, and I'll post them eventually.
Looking up wave development is probably something best left to pre-game prep, along with weather generation, but I think these tables are simple enough to use that they can be used at the game table in case of something changing suddenly, like with a weather control spell.
So we've eliminated momentary gusts of wind from blowing up huge waves, and we've made lakes and seas safer for small boats. So where do those waves come from on windless days? This post has gotten pretty long, so I think I'll save the answer to that question for another day.