Regarding the comments about the website notifying that the download won't work for Steam users, attached is a screenshot of the page that you buy it from - I can't see any mention of incompatibility with Steam versions.
My son had saved this link and downloaded when the 'purchase now' was added. Whilst I appreciate that the news section, and the mail shot had an update for Steam users, the purchase page did not.
I also checked the Steam store first, and as the ferry pack was not available I assumed getting it direct from the ShipSim website was the right thing to do.