Bobby,

to be waterproof, maybe you can use optical fibres.

If you not need high speed, you can use slow speed IR-LEDs and photo transistors and select a lower baud rate.

For arbitration and error checking the CAN bus need to check its own transmitter signal.
E.g. the photo transistor can also see its own transmitter LED or you use an AND gate to feed the transmit output into the receive input.


CAN data can only be 0 ... 8 bytes long. If you need to send more, you can send several packets.
Then e.g. use some bits of the 29 bit identifier to tell an ascending number for every packet to be sure, that they can be recombined into the right order.


Peter