There are many opinions on how to install a transducer, so there are quite a few guides below. Read them all, and then choose which suits you the best.
In-Hull Installations[]
Transom Mount Installations[]
The trick I have used when drilling external holes in a boat where there is wood is to locate the item first,drill the hole to the proper diameter,mix up some resin and fill this hole completely first,(it soaks into the exposed wood), redrill and insert a hardwood dowel coated in resin,them mount the item using a sealant on the bolts/screws.