During this summer's vacation, i had to take a ferry numerous times with my Tesla 3. Once on the ferry, it is not allowed to stay in the car or on the car deck during the voyage. The first times, i experienced the alarm going off short time after departure, i suppose due to car being moved without driving and/or reacting to rocking due to waves. After some googling, i turned off the alarm, but this also means the whole sentry mode needs to be deactivated. At the end i never reactivated sentry mode, as i was taking the ferry to "town" and then back again after a couple of hours. At the ferry, the cars are parked really close, and ofcourse i ended up with a door dent due to some idiot slammed their door into my car.
So the suggestion is, a "fery mode" or "boaty mc boat mode" if you like:
Keep sentry mode on, but deactivate triggering by "moving without driving" and "rocking" (due to waves).
The elegant solution would be that the question comes up on the screen when driving on a known ferry pier: "Are you going to take a ferry"? and then if yes then it is activated, or even better let it activate itself on and off when the voyage planned by navigation shows the car needs to take a ferry. Otherwise just have it as an option under the security menue.
And ofcourse, when activated, the car animation on the left side of the screen will show the car on a boat.
At the same time, i understand from other forums that ferry transportation is not calculated in as "teleportation" when the car calculates remaining energy onboard vs when next charging is needed, this issue may also be fixed as a part of "ferry mode".
Attached just a nice picture of my car waiting for the ferry, near Sandnessjoen Norway