Once you have conducted a search, your results will be shown on the search results screen.  Using the filters option will help you narrow down the list.

Keywords:  Enter any keywords to filter with

Location:  Change the search location

Categories (Trip types):  Add or remove trip types to filter

Features:  Add or remove boat features to filter

Distance Radius:  Narrow or expand the radius from the search location

As you add / remove search filters, the results will automatically update.