I think there is something missing from your problem. If the rope is on the same level as the boat then the boat is moving at 3 feet per second. I expect that there is a dock a certain height (say h feet) above the level of the water as in the diagram.
In the diagram x is the distance from the boat to the dock and r is the length of the rope, both in feet. h is a constant height, r and x are functions of time t, r is decreasing at the rate of 3 ft/sec and you want to know the rate that x is decreasing when x is 30 ft.
The triangle formed by the dock, the water and the rope is a right triangle and hence, by Pythagoras theorem,
h2 + x2 = r2
Differentiating both sides with respect to t gives
2 x dx/dt = 2 r dr/dt
Can you finish the problem?