Can I plug my GPS device directly into the UGPS topside to pass in NMEA sentences?

Not currently, unfortunately. Whilst there is a reserved interface for this in the UGPS topside unit as indicated at the following link...

https://waterlinked.github.io/underwater-gps/hardware/#internal

...it is not implemented at the present time. It is necessary to use the UGPS API, or one of the convenience scripts which we have written, to pass NMEA sentences from your device into the UGPS topside unit, as discussed at the following link.

https://support.waterlinked.com/en/knowledge/can-i-use-my-own-gps-for-greater-accuracy-with-ugps-g2