Power is defined as rate of doing work, P = W/t. Work done = F*s, so P = Fs/t. s/t is equal to velocity so P = Fv. As the boat is travelling at a constant speed, the thrust is equal to the drag, hence it is 1800N. It is travelling at 15ms^-1 so P=Fv and 1800 * 15 = 27000 W. The net force is 0, but it t mean the doesn't mean that the power is 0.
I hope it clarifies any concerns and if not, ask about anything you don't understand.