NMEA Docs

They’re generally paid, however the following is available online:

National-marine-electronics-association-nmea-2000-standard-for-serial-data-networking-of-marine-electronic-devices-main-document

NMEA 2000 Basics

250kbps CAN Bus

29bit identifier (ISO11783 standard)

8 byte data field

Open Source Libraries

The NMEA2000 library by Timo Lappalainen basically makes NMEA2000 possible for the masses (it’s a big and complex standard!):

https://github.com/ttlappalainen/NMEA2000