Where are ship tracking units generally placed on a luxury yacht?

Question

Are they attached to the hull beneath the waterline, or hidden somewhere on board? (for a story I'm writing)

Answer 1

It's no secret they are mounted in the bridge alongside all the other electronic gear, and maze of wires and antenna. In today's world,  it's almost impossible to get very far in a boat without someone watching.   That is particularly true of mega-yachts.  Many ports have radar like an airport control tower, and cameras everywhere.