Way back in 1987, GW released a box supplement to Warhammer called The Enemy Within: Death on the Reik. In addition to the main book, it contained a 16 page booklet called River Life of the Empire which had as its centrefold a spread on river boats. There were half a dozen profile and plan views of various boats from a rowing boat up to a large river boat, with the plan views showing interior arrangements. No scale unfortunately but that could probably be worked out with a bit of thought. It may be what you're looking for.
If you can't find a copy, I could try scanning it for you.