Repurposing a sonar transducer from a fish finder?

would it be possible to repurpose an old fish finder transducer to make a ice fishing flasher? If so, what extra hardware would be needed other than the arduino and the old transducer?

Or even use transducers for other sonar related projects?

Most probably you need a powerful driver module for the sender, and an amplifier and filter for the receiver part. Such circuits are part of ordinary US sensors, but at least the sender part will be too weak for submarine applications.

For the application code you can try the examples for the US distance sensors, but I don't know whether they work well in water, and for detecting fishes. I'd ask such questions in a fishing forum, then come back for implementation details.