Twelve-Foot Swells?

Checking the ferry schedule to see if there were any earlier bookings, I ran across this. Twelve-foot swells? Just how keen are you to get across?

Interislander - Service Alert

Wednesday 22 March: The 3:45 pm (ex Wellington) and 8:35 pm (ex Picton) Aratere sailings are going ahead as well as the 8:30 pm Kaiarahi sailing from Wellington tonight but it will be rough. Swells are likely to be over 4 metres. You may want to reconsider your travel if you get seasick. Full refunds are available and unfortunately, we are fully booked and cannot offer alternative sailings.  See Service Alerts for more.