Comment 16 for bug 1722376

OK, allows_seafaring now works. BTW I found that this function is almost nowhere used. f.e. here is place where it should be used:

http://bazaar.launchpad.net/~widelands-dev/widelands/trunk/view/head:/src/wui/fieldaction.cc#L421

But try to grep the code for get_port_spaces().size(), there are probably couple of places where this is used as "estimation" if map allows seafaring...