The rule I learned is that the height of the wind wave plus the height of the swell should not exceed the frequency of the swell.

If you look at the Thursday prediction for the Neah Bay area below, it would be a good day to keep small boats at the dock.

The Friday prediction might be fishable, although lumpy, if the swell frequency is more than 9 seconds.

THU
NW WIND 10 TO 20 KT. WIND WAVES 1 TO 3 FT. NW SWELL 6 FT AT 7 SECONDS.

FRI
NW WIND 10 TO 20 KT. WIND WAVES 1 TO 3 FT. NW SWELL 6 FT.