I found this in my notes, and then tracked it down to this link at the NOAA marine weather page, which has several topics of interest to mariners.
Very roughly, periods of less than 9 seconds are waves, and those with periods greater than 11 seconds are swells. I believe the average wave period over the ocean is about 7 seconds.
It is of course not gospel, but certainly a good guideline and perspective, since the forecasts just give the heights and sometimes periods, without such an evaluation.
Below the picture is the text that they present to describe the table.
"The danger presented to a vessel is a function of wave steepness as well as wave height and is unique to each vessel. In general for small vessels, for a given wave height the danger increases as the wave period decreases. Below is a table under development within the National Weather Service to assist forecasters in identifying sea conditions which may be of danger to vessels with a closed cockpit configuration of ~100 feet or less. The table is intended to be instructional only and the danger presented by waves to your own vessel may be quite different"
We can compute wave length from the period (with certain assumptions), and from the height and length we can compute steepness. See Modern Marine Weather for more discussion.
If we approximate the Length from the average sea way of the periods shown, then we can get average lengths and average steepness, being the ratio of Length / Height. Waves generally break at Steepness of 7, but they look terrible and are certainly dangerous at Steepness of 10. It seems from this table we calculate below, that they have called dangerous a value of 25 or less, which makes sense. It is not clear why there is not more red at the shorter periods.
For a truly small craft, this graph should, I believe, be red for the shorter waves, with Steepness up to 15 or so. We will have to look into this some more to see their logic.