So is the SeaTalk protocol similar to RS-485?
No, RS-485 is a multi-drop whereas the Seatalk bus is similar to RS-232 as far as voltage levels are concerned but devices on the bus can 'talk' at anytime - collision detection being a case of abandoning the currently-transmitting (or receiving) packet and waiting a 'random' interval before trying again if the bus is in conflict (i.e detection of erroneous characters would be one example for 'abandonment').

The 9th data bit is a 'command' ('set' on the first character of a packet but subsequently 'cleared') and sort-of fits the parity bit in a standard serial comms stream - certainly for 'position'. In terms of readability one could configure the serial port (UART) for 8-bit plus parity and somehow read-but-ignore the fact that it is supposed to be a parity bit but instead just read it and treat it as the indicator of a 'command'. Not sure if a UART would permit this as defining it as a parity bit would constantly create errors as the parity wouldn't be corect on most characters received.

I shall download the PIC EUSART datasheet for a shufty.... thanks for the pointer.