The biggest problem I've had getting charting software
to work with a GPS is poor serial<->USB adaptors (& their drivers). Get a high-quality one, not the cheapest. FWIW, I use a KeySpan 4-port serial
to USB, but there are other good ones.
Also, different GPSs use different "sentances" to send their data, & both MaxSea
& OpenCPN sometimes need to be tweaked to listen for the correct ones. For instance, I think there are at least 5 different sentances that can include position info. You have to look at the input stream from the GPS & match it in your nav program. Most GPS sentances start with $GPxxx & the trick is getting the xxx correct. Make sure OpenCPN is looking for the same sentance that your GPS is spitting out.