The most straightforward way is to express the x- and y-coordinates of the boats as functions of time, then express the square of distance as a function of time and equate it to 100. You'll get a quadratic equation in time. However, writing this in exact form is cumbersome. This method is suitable if you only need a numerical solution for concrete distance and not, say, to express time as a function of distance.
A simpler solution that also allows expressing time as a function of distance (or vice versa) is to use the law of cosines.