Like Smac999 said, any 3-wire cable will do. However, there are some advantages on using 'real' Seatalk1 cable. First, the Seatalk1 cable is shielded, which helps to avoid data loss due to external
noise. Second, is is very hard to obtain a 3-wire cable with the right color coding of te wires:
red-12 Volt plus
yellow-Seatalk1 data
Blank shielding-common minus
There comes a day when you are glad you used the correct wire colors!