If your boat is in the water with an external transducer, you're SOL. Your only realistic recourse is to temporarily connect a transducer with a similar instrument connector and drop it over the side.
If it's a shoot thru, check for oil or alcohol in the transducer well (whichever) and be sure the face of the transducer is "wet"..no bubbles. Top off if necessary. You may still need to swap transducers, as above.
I'm beginning to think you've a bum transducer...maybe it was a victim of a wayward travellift strap.
All (almost all) transducers are made by one manufacturer...Airmar. They are ceramic or sometimes crystaline, but hardly ever magnetostrictive anymore and so cannot be checked with an ohmeter.
What you can do however, if you're so inclined and have a lot of time on your hands and can get to your transducer, is to connect the transducer connector to the input of a small audio amplifier and headphone of the sort sold by Radio Shack. If your transducer is good, you'll be able to hear it in the phones or speaker if you tap it. This is vaguely related to the sort of microphone used in hydrophones. I should say that this doesn't really test the functionality of the instrument..