The Arctic seas are unforgiving. The conditions are likely to change from day to day and even from hour to hour. You can ask the ferry operator, but with a 4 hour one way trip they probably don't make more than one trip per day so they'll be getting their info second hand too.
That particular ferry route is on open, unprotected waters so it is possible that the conditions will be quite rough. However, you can get a weather forecast from http://en.vedur.is/weather/shipping/coastal/, which today looks something like:
The area around Dalvík and Grímsey currently has light winds, which may mean that the seas are relatively calm.
If you want to go anyway, I would suggest preparing yourself for the feeling of seasickness by whatever method you prefer. Take an anti-nausea pill, don't eat too much beforehand, and keep looking outside (with a view of the horizon, but don't stare at the horizon).