I looked at GPX - my idea is to do the minimum of data processing when recording the data, and to use GPX the NMEA strings would have to be decoded first. In addition each message in a stream could refer to a different object (ship) so these would not be routes or tracks, but a stream of not necessarily connected points. Also not all AIS sentences contain position data, but they all contain the MMSI of the source object. I'm not sure if there is any way to replay data like that.