Seatalk is powered by one of two means.
1: from your autopilot
. if you take the removable cover off your pilot there is a switch there (small one, upper left) that can power the seatalk system. there is also a fuse for it there.
(note, if this is switched off, don't just switch it on, as it is probably powered by the system below, and it is not good to have two power sources).
2: separate power cable, Seatalk cables
are blue with black (backbone) and white with black (Spur) also yellow with black but that is for an interface to lower versions of seatalk. The seatalk power cable is red with black, you will find it normally plugged into a "5 way connector" or "T" but I think it is possible to plug
it into the back of an instrument as well. (not sure on this, just seems logical).
Raymarine recommends a 5 amp fuse to this power cable.
Hope this helps,