"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
Conversely, even if the English description is simply “tall”, the boats could be actually very tall. The same also applies in Chinese that 很 is often a dummy word.