Problems like this call for ol' Pythagoras.

The boats are 240 feet apart. That is constant. Therefore, dx/dt=0

One is travelling at 25 ft/sec and the other at 15 ft/sec. 25-15=10

dy/dt=10 ft/sec.

When x=240 and y=450, then D=510

Differentiate:

Check my figures. I have done this in a hurry.