Draw the right triangle formed by the boat's horizontal plane of motion, the height of the rope's attachment point (through a pulley, maybe?) above the water, and the hypotenuse being the rope's length between this point and the boat.

Label the height as "h = 2", with dh/dt = 0. Label the distance between the boat and the dock as "x", with dx/dt = -0.1. Then the rope's length is clearly D = sqrt[4 + x^2]".

Differentiate the equation for the rope's length with respect to time "t". Plug in the known values, and solve for dD/dt.

If you get stuck, please reply showing how far you have gotten. Thank you!