Connecting NMEA 0183 devices directly to a PC without isolation is not recommended as this can cause ground loops which will damage your PC and your NMEA 0183 devices. You may be interested in an Actisense USG-2 which provides isolation on the input and output, keeping your devices safe from ground loops. The USG-2 also has a USB lead, making connections to a PC quick and simple.
A slightly lower cost alternative that we recommend as an absolute minimum means of electrical protection when interfacing NMEA 0183 devices to a PC is the Actisense OPTO-4. This will provide isolation to the PC's input but not the output. If your PC does not have a serial port you will also require a USB to serial adaptor: