Hi NJAdam,
Offshore over 15 ft. ... manageable but not pleasant.
The real thing to consider is not just the height of the seas, but the wave period and the "makeup" of the wave (i.e. gentle rollers or chopped up tops).
You can have tremendously large seas with no chop on top that are spread out that any boat can handle.
But then you can have high seas that are tight (and add chop on top). Then it gets serious.
I hope I didn't confuse the situation but it is more than pure height to me ...
Wave formation/makeup and wave period can make 3 ft. seas more hazardous than 10 ft. long period swells.
Capt. Ed