When searching in the app for BoatUS marinas, you can elect to apply filters to your search the same way you would if you were looking for a marina that had a fuel dock.
From the Explore main page (or Explore button on the toolbar) choose "Current Location". Then select "Add Filters" and tap the BoatUS discounts you are looking for. You should also fill in the dates of your trip and any other amenities on this screen. (See images below for reference).
Note: You can also click "Add Filters" directly from the Map button.
Once your filters are chosen, click "Apply Filters" and your search will come up with the marinas that fit your search criteria and BoatUS discounts. (See an example search below).