Using google maps to tell the robot where to go

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I think you're really asking two questions here: 1) How do you convert a geographic coordinate into a coordinate in your boat's map frame, and 2) How do you navigate to a point in your boat's map frame. The details of the answers will depend a lot on your setup, but I'll give some high level ideas for each.

For 1:

If you are okay with doing some manual work every time you want to move your boat, you can convert the lat-long coordinates of the point you want to move to into UTM coordinates. Assuming you are localizing your boat with GPS, you should be able to convert the UTM coordinates into coordinates in your boat's map frame.

If you want to be able to click a point on a map and automatically have your boat navigate to that point, a good starting point would be to look at the ROS packages RVizSatellite and mapviz, which can both be used to display maps (maps are generated using map tiles). Something to note though is that Google map tiles are only officially available through their API, which you'll need a key to use. There are free map tile providers though (like mapbox), which you can use with RVizSatellie & mapviz.

For 2:

Once you have the coordinates of your desired position in your boat's map frame, you should be able to use the existing ros navigation stack for sending commands (see tutorials here).

You can use Mapviz with plugin Point Click Publisher.

To set your goal to Navigation Stack you can use gps_goal. But Point Click Publisher publish point using geometry_msgs::PointStamped and gps_goal listen for geometry_msgs/PoseStamped or sensor_msgs/NavSatFix. You need modify gps_goal or convert message to required type for your purpose.

To display Google Maps in Mapviz you can use this solution.

Edit 1: Now gps_goal using NED reference frame, according REP-103 convention

In my opinion, it is better to use Cartesian coordinates, since the UTM does not coincide with the maps provided by the Mapbox and Google. I use GeographicLib to convert WGS84 - Cartesian