I have a couple of zipcode maps. The map shows local zipcode regions with a numbers overlayed that show total customer invoices for each zipcode for a specified date range. I used relationship filtered portals for this as it is implemented in FileMaker 10. In FileMaker 11 or newer, I'd use a filtered portal to get the same result with a less complex relationship graph.
Put a field in your Vessels table that stores the slip number.
Then put one row filtered portals on top of your marina image to display the vessel name. Give each such one row portal a filter expression such as
Vessels::Slip = "D-30" to filter for a specific Vessel record.
To pop up additional info about the vessel from this layout, there are two options you can play with: Use a calcualtion inside the ToolTip setting for a field in the portals to display additional information. This info will then pop up automatically when you hover the mouse over that field.
Use button setup to turn the Vessel Name fields into buttons that use Go To Related Records with the New Window option to pop up a small window displaying additional data about the Vessel when you click that field.
Thanks for a starting point. I'm sure I'll be back to forum for more sage advice.