thru hull
getting a new thru hull transducer with a fish finder will prob add $100 to the cost, you may be able to use your old transducer with a fish finder. with a thru hull like you have, you can get a fishfinder with speed, temp also. before throwing the old one away, mmake sure that all the wires and connectors are clean and the wire not corroded, cleaning the contacts just might bring the old one back to life.