I did my own survey a while back. Here is what I found for 6mm ships:
6mm (somewhere between 1/285th and 1/300th scale)
Heimskringla Longships (58mm-122mm long), now manufactured by Heroics and Ross), make some brilliant 1/300th scale Viking longships, the smallest of which could possibly convert to 1/600th.
Irregular makes 6mm Longship.
Eureka (Grumpy) Miniatures makes several types of ships in 6mm, including Dhow and Chinese Junk.
In 15mm, I found the following options:
Old Glory Shipyards makes Medieval Cogs, Viking Longships, and Saxon and Norman Knarrs.
Museum Miniatures makes a Chinese Junk and an Arab Dhow (both are 80mm long).
Essex makes a Viking boat, Dark Age leader's boat, and a Medieval boat (all are 80mm long).