For any underwater caulking I would use 'Life seal' caulk- it is a silicone with adhesive and it is easy to clean up which makes for a clean install. Forget the starboard, mount the transducer right to the hull, just measure 3 times and drill once...Route the wire straight up from the transducer and if your hull allows, drill the hole above the waterline as this will probably be the biggest hole and will go all the way through into the engine compartment.
Use a clamshell or similar thru-hull fitting and caulk it well where the wire enters. Good to go.
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