Just set it there to make sure you get good satalite reception before drilling holes and mounting. Have people sit/stand in normal positions. It shouldn't be any issue, but always best to check.
If marking fish/structure for the most accuracy, place it near the transducer.
After setting it (unmounted) where you wnat it, drive the boat around a bit. If the antenna gets blocked by trees/building/etc, it's quick to lose a signal. They usually fixate on 6 satellites so all they need is to lose one and you'll miss your signal.