This is done like other Pythagoras related rates problems, only you have a 3-dimensional component.

Differentiate:

Now, you have dx/dt, dy/dt, and you can find D once you know x and y. Find x and y after each has traveled 10 seconds. For instance, at 60 km/h, how far does the boat travel in 10 seconds. That could be your x. Carry on?.