I would answer 'There is no way to tell'. There are some boats. There might be one, two, three, ten, or one hundred. We don't know how many. They are in (some) rivers. There might be one, two, three, ten, or one hundred. We don't know how many. We don't know which river the questioner is asking about. We also don't know if there is a boat in that specific river. As to 'what's the difference between 'there is a boat...'and 'there is one boat.....'' - normally 'a boat' would mean one boat. But it could also mean 'there is at least one boat'. You can only know which one with context. e.g. 'Can I park in the parking building? Will there be a car park free at 4pm? // Yes, there will be a car park free then. <- In this case, it means 'at least one'. You are only interested that there is at least one for you.

Respected Madam, According to your question:- is there a boat in the river? describes that the sentence in Interrogative form or we can say that the sentcnece is a Interrogative Sentence As I have cleared you before. These type of sentence is used to asked a question which can be answered either in Yes or No you are free to answer it according to your common sense. secondly there is no difference between both the sentences "There is a boat ------------ There is one boat as in the first sentence "a" and in the second sentence "one" is written which denotes a singular number but the first sentence is absolutely correct and written in a correct gramaticial form.

Yes , there is a boat in the river. At least one, but more than one as well. The use of the word " some " is a nonspecific way to say there are more than one but no way of knowing how many exactly. It needs to be taken in context of the discussion.