Hi Gordon, Does the unit have a built in Ariel, or an external one. If it's external, any position inside a cabin as long as it's within easy reach and viewing angle will suffice. If it has an internal Ariel, then location is a bit more critical as best reception is with a clear view to the sky, so if it is 100% waterproof an outside of cabin location will be best.
Sonar. If you have a transom mount transducer, the closer to the centre keel gets the least bubbles whilst on the move, but best about 12 inches away from the propeller, with the bottom of the transducer about quarter of an inch below the bottom of the boat where its fixed.
If it has a shoot through hull transducer, again somewhere near the centre of the boat at the rear, but only through a single layer of fibreglass, and use the proper resin. The aim is to keep away from any air bubbles under the transducer, otherwise you loose the picture for a while. Hope that's of some help. Arthur