The OP confirmed that the same setup worked fine on another computer, so it's not necessarily the
hardware.
Here's the latest official driver for Mac, if you don't already have it
Since there's data, but it's not correct, I believe the driver's working ok but speed setting may be wrong. First thing to try - change the expected speed in
OpenCpn to 9600 and see what happens. [and 38400 too as per Jean]
What is supplying the GPS
serial signal? Check its settings for speed and format.
I'm not a Mac guy, so I have nothing further to suggest. Maybe hit the Mac
forums to find out what they recommend as the best USB-serial cable for Mac.