I believe this is an example of Bernoulli's principle. That as the flow of the fluid increases, the pressure decreases. This is the same principle involved in generating lift on a plane's wings.
I suspect that the boats have to be within a certain distance for this event to occur. Consider the flow of water if two boats are relatively close together, moving in parallel. Between the two boats there are two oars moving the water backward, while on the outside of either boat there is only one oar acting on the water. Therefore, the water between the two boats is moving faster than the water on the outside of either boat. This means the pressure is lower between the boats than on the outside of either boat. Since the pressure is higher on the outside than on the inside, the boats will slowly move closer together. And the closer together they get, the higher the velocity of the water between the boats and so they should draw together faster as the distance between them decreases.
when boats move in water then due to friction between water and the bottom of boats some heat is produced which makes the water between boats lighter than the water around them which is at high pressure.due to high pressure around, the boats are pushed by the water around and are pulled towards each other