If you code the RSS as you might expect
Tide times and heights not available for -tide-times
Warning: simplexml_load_file(http://www.tidetimes.org.uk/herne-bay.rss) [function.simplexml-load-file]:
failed to open stream: HTTP request failed! HTTP/1.1 404 Not Found
in /home/oikplug/public_html/wp-content/plugins/oik/oik-tides.php on line 43
and if you click on the link in the second shortcode you’ll get a 404
So why does it work for Portsmouth and Chichester (Harbour Entrance) without the -tide-times suffix? See the oik tides plugin page
Thanks to Jason Hollingsworth for pointing this out.
PS. I think the explanation is something to do with redirection processing at tidetimes.org.uk working for Pompey and Chi but not other, perhaps newer to the site, locations.