Glad to hear you found the cause.
Most electronic devices have a water Ingress rating, which means, how much water they can "digest". I have a B&G Zeus 2 at the helm (on the starboard side, behind a window, which can't be opened on the Santorin) and its water Ingress rating is IPX 6 and 7. I dont know what Your Furuno plotter has. But according to the protection it is built one can "stress" it.
Here is an IPX list from the web.
IPX0 – This means it’s not water resistant at all. Think of what happens to paper when it gets wet. It’s a mess and your novella is ruined.
IPX1 – This will protect a device from some water drops that are falling vertically on said device. I guess it’s possible for this situation to actually occur but so is winning the lottery. If you or someone you know has won the lottery, sharing is caring.
IPX2 – This will protect your thingy from some water drops when the device is tilted up to and including 15°. If you have your device at 16°, you are screwed, sorry but there are limits in the world.
IPX3 – Getting better, you can now spray your gear up to 60° from the top of the device. So that’s cool, I guess.
IPX4 – Now we are getting somewhere. This will keep “yo ish chill” from splashing water from any direction. So if you drop some cubes of ice into you scotch and it splashes, there is no need to worry. Well, except for those drops of scotch that didn’t make it into your tummy.
IPX5 – (this is where the super soaker is allowed). Will protect your stuff from water jets at any direction. Spray away kiddo, spray away.
IPX6 – Protects from powerful water jets. So if you modified your super soaker with an air compressor and an aftermarket tip, your stuff is still safe.
Everything after this is now water proof.
IPX7 – Full splashdown accepted captain. If you drop your device in water up to 3 feet (1 meter but this is ‘Merica and we don’t do the metric system. USA USA USA) your device is still going to work.
IPX8 – You can protect your device in water over 3 feet. This is for your stuff at the bottom of a pool, lake, or even a super deep public hot tub. What’s up with that one couple that never gets out of the hot tub?