The ferry from Vaasa to Uumaja has turned back due to bad weather. Is there a way of following where it is, expected arrival time and other such information?
You can follow many vessels worldwide through vessel finder websites, such as vesselfinder.com or marinetraffic.com. The websites are free for basic use, not unlike airline tracking websites. This will allow you to see where the vessel is right now.
Specifically, you can follow the WASA EXPRESS. It is apparently due in Holmsund next at 2018-11-29 12:30 LT (UTC +1), having left Vaasa at 2018-11-29 09:03 LT (UTC +2). I don't know how quickly marinetraffic will pick up in case plans change, but when it has physically turned around you should be able to tell from the live tracking soon enough.
Edit: According to a comment by vidarlo, Destination is sent as part of AIS messages, so Marinetraffic (and other sites) will pick up as soon as the bridge changes destination on their AIS instruments.