Rent 5 large boats, 1 small boat.

We can solve this problem by keeping chickens and rabbits in the same cage in the Olympic Games in primary schools. First of all, suppose everyone takes a big boat. According to the meaning of the question, there are 4 people in the ship, but there are 23 people. Then divide the total by the number of people in a big ship, that is, 23÷4=5? We found that there were three people who rented five big boats without a boat, but according to the known situation, we found that three people just took a small boat. Then, in view of economy, we only need to rent five big boats and one small boat.

To sum up, the correct answer to this question is: 5 big 1 small.