About 6 years ago I wrote a Python script for a customer in the marine sector. It pulled the GPS location from an onboard vessel system and updated the vessel location using the GPS coordinates. This then allowed the Meraki portal to show where the vessel was.
You don't actually have to do it onboard though if you are using something like marinetraffic.com - you can use their API to pull the vessel location and update it - all from shore side.
The Meraki API has changed since I did it. Basically, in the address field of an MX appliance, you can put in GPS co-ordinates. Start by doing that in the dashboard, so you get it in the correct format.
Then use this API call to get that data. This will let you see what the field names are.
https://developer.cisco.com/meraki/api-v1/get-network-device/
Once you know the field names for the GPS coordinates, you can use this API call to update them whenever you want.
https://developer.cisco.com/meraki/api-v1/update-network-device/
From long ago, this was the key line of code that did the update - but I doubt this will work with the current API version. Hence the exercise above.
dashboard.devices.updateNetworkDevice(netId,serial,lat=lat,lng=long)