I have now spent some time looking into this, and cannot reproduce it: I have added an assertion to check the registration of passenger destinations to see whether they are on water other than a depot or a stop (as some passengers actually work at docks and shipyards and have those as their ultimate destinations). I have not been able to trigger this assertion. Are you able to upload a saved game in which this can reliably be reproduced (i.e. passengers having the sea, being not a dock or shipyard, as their destination)?
Thanks for following up. I spent a couple free weekends trying to repro. Not seeing any weird water activity. I am seeing however cities with very large orange dots (which i assume is passenger desired activity) in strange, far away places where there are not necessarily nearby attraction, or there are, but it doesn't seem realistic to ever give them a way to get there.
See the link below for a save file It's big (15MB) given the map size but I think this is an issue with such maps that the AI isn't thinking clearly about destinations. You can also see how little activity there is for nearby destinations. for big cities (20,000+ residents) but there are these very few, very far away destinations that seem to be where all the people want to go, unless i'm misunderstanding what the destinations map is telling me.
Some good cities to see these examples are:
Spiningford (look at the very top left of the destination map)
MickleGreeningly (towards middle of the map)http://happy.lanukah.com/nextcloud/index.php/s/wKunN77XmaDD1Lk