Any slow cure two-part marine grade epoxy will do. The key is in the slow cure part. Stay away from anything with less than a 1 hour cure time, the 12 and 24 hour cures are best. Try to keep the air bubbles out while mixing it and always pre-test the installation location first.

I would think that plumbers putty would have small air bubbles in it and so would not work well under a transducer. Maybe to help build a dam to hold the water in but not under it. I would use a plastic storage bag with some dry dirt or sand in it to hold the transducer in place while testing.