And I've been on the ferries too
One trip was a smooth as a millpond and comfy as heck, few days later it was as rough as hell, people throwing up everywhere (literally) and staggering around like drunks
(fortunately not me!)
I'm just saying in my opinion it wouldn't be a sensible way to travel for a heavily pregnant, feeling crap, lady.
Why I put sensible in a comment about KP though I just don't know
Im lucky enough to of never been on a rough crossing ..but believe in winter it's awfully rough.. I've only travelled in the summer months when it's a lot smoother..
I'm glad I haven't been on a trip like the one you described it must of been awful