"These boats are tall."
很 (hěn) is often a grammatical link between a noun and adjective, which translates as "is/are" rather than "very". If these boats are very tall, it would more likely be: 这些船非常高 (非常 = fēicháng = very).
You can read the tips and notes from the DuoLingo lesson called "Greetings 2", or here is a longer explanation from AllSetLearning
I have already left a comment on the opposite translation thread. I am not sure in this particular case Japanese and Chinese use same logic, but in Japanese the word could refer to an item being expensive, not tall. So it is either 'big boats' or 'expensive boats'. Either way it is confusing and calls for revision...