I would think it need up top of the mast and as mentioned have one lower down. The lower light can help when walking around at night. So, it not hurt.
But, I have always assumed you need a light on top of the mast so airplanes not run into it.
Well maybe not.
You could get a Vessel Safety Check by the USCG for free and they give the specifics and they not going to write a ticket if something missing.
Here the link.
http://nws.cgaux.org/visitors/ve_visitor/index.html
Read and then click; more information; "SafetySeal.net"
Then click; "I want a VSC" and enter your zip code.