I think the biggest factor in what you use to "seal" the puck against the interior hull wall ... is AIR BUBBLES. Air bubbles are NOT GOOD, so you'd want to use something that did not have them. And even if you use the epoxy, you'd have to make sure to stir/mix it slowly, so as to not induce air bubbles into it.
I found this discussion, that might interest you : Transducer Mounting Epoxy
... cp