According to:
http://www.faketitles.com/html/principality_of_sealand.html
http://en.wikipedia.org/wiki/Principality_of_Sealand
http://geography.about.com/od/politicalgeography/a/sealand.htm

...it's not recognized at all.

I didn't find that name in any list of sovereign states or countries, nor even on this list of states with limited recognition:
http://en.wikipedia.org/wiki/List_of_states_with_limited_recognition

I also found this one:
http://principality-of-sealand.eu/about/geschichte_e.html
...though the domain name suggests that it may be biased.