Is there a reliable source for Electronic Navigation Charts, where they get updated regularly? I am able to find for American waters on the NOAA website, but not for other regions.
Here's a couple I was able to source. Listed in order of relevancy/trustworthiness.
http://ris.vlaanderen.be/html_en/vaarkaarten/index.html -Belgian government ENCs
http://ienccloud.us -inland USA ENCs, from the Army Geospatial center
https://www.aragon-technologies.com/en/starnavpilot/download_free_nautical_charts.php -Aragon Technologies has compiled a large list, especially for European waters
http://www.opennauticalchart.org -literally a world ENC online map. Downloadable in a variety of formats.
https://sites.google.com/site/nauticalfree/ -user compiled but seems to be thorough
https://sourceforge.net/projects/opennautical/ -sourceforge open source ENC downloads
For a non-open data option, I work at UKHO, who publish over 15000 ENCs and coverage is fully global. I believe UKHO are one of the only hydrographic offices that maintain global coverage. As you say, most countries will chart their own territorial waters only.
Updates are applied continuously and pushed out to users - I believe that you can download weekly updates (or receive them on disc).
In terms of accuracy, all the data are official and meet the requirements of the IHO standard for navigation data.
To buy ENCs, details of distributor near you can be found here.