How can I get the Navionics App for iphone to utilise a third party gps
From the frequently asked questions page, it appears Navionics has not tested third party GPS devices:


  Q. If I attach an external GPS to my iPod Touch 2nd Generation, will I locate my position on the Navionics app?
  
  A. Navionics has not tested this thoroughly enough to guarantee a good performance.


The answer suggests third party GPS devices should just work, but may not be reliable.