The capacitor is to block DC current while passing RF. If you are keeping your SSB and DC 'grounds' separate, as you should, and if you should decide to ground the set, but you don't need to, the capacitor will keep the DC isolation between SSB 'ground' and ship's neg.

For a receive test, how about RAF volmet: 5450 kHz, 11253 kHz. 24 hours service.

You will find lots of freq's and times here:

http://www.yachtcom.info/Frequencies.htm