Comment 8 for bug 1234725

There was a large Building space icon combined with an anchor in Settlers' II build help to signal that a port could be built there.
If that icon did not show up for any reason, I think there was no further hint.
http://jajozglowy.pl/gry/solucje/s2/flota1.jpg

Personally, I'd prefer to hide the port spots if covered under trees, stones or buildiings.
It's a nice extra challenge to disvover them.
After all you can always find the spots by letting a ship search the coast.