There used to be a link to a spreadsheet on this site with a roster of all current and former C36IA members, with hull numbers and other info, but I haven't seen a link to it for years.  
I have an old copy on my computer that I downloaded back in June 2011, but there is no listing for #688.

Enter   Coast Guard registrations.com then scroll down to vessel registration. A form will pop up, just put in the vessel name and a history of the name will appear.

There are ten boats with the name Bulletproff. According to the USCG Hull number 688 was registered to Robert Young in Gulfport on 6/27/05,  CG Number 910752, which expired on 6/30/06. The last CG registered owner was Sean Appleyard. You can find all this information in the CG Site I mentioned above. The present owner may not have registered the boat with the Coast Guard after 6/30/06, and only registered it with the State. If you use the complete Hull Number in a query to the State where it was registered after that date you may find the present owner if it is registered in that state. Some states will give you this information at no charge, but most states do charge a small fee.  If the ownership was transferd to another state the first state may have the information where it was transferred to.  Good luck and let us know how you make out.