raisepe;
Unfortunately you will create ground loop with CAN-BUS Shield V2.0. Only question is how long. If your NMEA2000 bus power, your device and engine ground are nearly same point (<50 cm), there is no problem. If you feed your bus on e.g. chart table and have your device 6 m (in wire length) away on engine, then you ground your bus again near engine. In that point situation is bit better if your device is near bus power point, but then you will have long measuring cables from engine and may get noise to them.
Making CAN shield by own is not rocket science. I made my first shield according to https://github.com/ttlappalainen/NMEA2000/blob/master/Documents/ArduinoMega_CAN_with_MCP2515_MCP2551.pdf . Change MCP2551 to ISO1015 as on document https://github.com/ttlappalainen/NMEA2000/blob/master/Documents/Connecting_hardware_to_NMEA2000.pdf and you will have safe isolated shield.
And think to go to Teensy 3.2 - smaller board, more power more memory, internal CAN controller. You need only transceiver,