Just saw this, pretty much already been said. I have made this trip many times over the years on the MV Coho, a 340 foot ferry, sometimes it was dead flat calm, many others with a short high sea. Hard to tell from a large ship but I would estimate at least 10 to 15 feet as mentioned earlier. Glad I wasn't in a small boat. Suggest you watch the weather and tides carefully.
I would guess a flood tide would be the best time to cross? Be carefull!!!