Not true. Almost all "pucks" are epoxied to the inside of the bottom of the boat. Often, they are in the engine compartment. If you have a cored fiberglass hull structure, it's not going read well. If you have a solid fiberglass hull structure, it ought to do well as long as the epoxy is solid and thorough with no air bubbles or gaps.
Usually, the best advice for a step bottom boat is to put the puck/transducer in front of the foremost step on the boat, as close to the bottom of "V" as possible.