Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
I would advise you first of all, to dismount the Garmin from your boat, and take it to a computer.
Then get Garmin Express onto the computer (it works on both Apple and Windows PC, in different versions meant for each one, specifically.)
When dealing with an update for your Garmin device, it is best to simply download, install, and use Garmin Express.
and (if you have a Windows PC) click on the big blue Download button at center bottom of the page. (If you have an Apple, click on the nearby Download for Mac link.) And download Garmin Express' installer to your computer, save it, then double-click upon it to install Garmin Express.
Then start the program, Garmin Express. Then connect your Garmin device. Then use Garmin Express to update your device.
Let me know how that goes, please. If it works, I can close this case if you let me know. If it doesn't get the device working properly, let me know in that event, and I will have further suggestions for the problem. Thanks.