??? How about (speculative, this, as I haven't tested it)
Paint sea tiles where you want this to happen with a region. (Obviously you would have to do this on each map).
Use Shaz's script which enables regions to call a common event. (Script
here)
That common event does the change graphic/board boat thing.
Reverse the process for getting off.
Paint coastal tiles with another region which calls a different common event.
This could get pretty cumbersome, so you might want to use a pier/landing jetty as the only place on each bit of coast where this could happen. Then you would need Galv's Vehicle On/Off Location script to restrict the places where boats can land.
I'll have to go hunting for the link for that one.
EDIT
And here it is
http://galvs-scripts.com/category/rmvxa-scripts/on-map/#post-20