The Parker-Dawson will be the final boat I own and restore. I have had a lot of experience securing thru-the-hull transducers for fiberglass boats and I am now replacing the 1970's model with a current depthfinder and I have a few tips that have worked for me. First, unless you are going to be fishing, buy a depthfinder only, and not a fishfinder, as the readings are not as accurate and I worry about getting a quick accurate depth in an emergency situation. Second, having only experience with placing transducers in fiberglass boats, I have found that the "puck" type  is the best.
Opinions vary, but there are a few things that are common sense. Some people say to mount the transducer toward the rear of the boat. I don't agree, as there a lot of other things toward the back that interfere with the readings. Most of all, the engine vibration that can screw up reading. I like the furthest point forward, deepest part of the hull, where water is always covering the outside.
I have read articles where people put all kinds of boxes around the transducer. (PVC, homemade boxes, etc) After reading about what they have to go through and the potential for leakage, it seems a bit like over doing it, especially transducers surrounded by mineral oil. The puck is really not secured, and I can see where floating around in the oil could cause problems.
I think the simplest way is the best and with a fiberglass hull, flexing does occur and epoxy, although strong, can pop off, even with  clean surface. Also, the action of stirring two part epoxies creates bubbles that are not visible during mixing but come together during the curing process. For all those reasons, I have always used silicone. It seems like the least popular way, but it has never failed me and I can remove it without damage the either the hull or the transducer. Having a clean surface is most important and I have used a clear silicone with no problems. Some say that they have have had problems with the silicone not curing completely, but it may have been the brand. 
Once I have found the right spot, I make sure it is clean and dry. I then squeeze out a good quantity of silicone to the hull and make sure that there are no bubbles. I don't coat the puck, as this can cause bubbles when the silicone is spread on. I then set the puck into the center of the silicone from a slight angle to avoid any bubbles. I then push down, almost to the point of the puck touching the hull. The silicone should be sufficient so that about 1/2 inch protrudes from the side. If it appears level, I leave it alone. If it is crooked by eye, I use a level to get it as straight as possible. I do not wipe away the extra, as after it clears, using a flashlight behind the puck, I can see any bubbles. Wiping the excess is messy and makes it harder to see through after it cures. If there are big bubbles, start again. I have never seen bubbles.
After that I leave it alone and let it cure. I am sure there are a lot of people who don't like this method, and what works for them is great. This is just been the best way for me to connect the puck.

On my PD, I mounted mine in the storage locker, the furthest one aft on the V-berth, that goes all the way the the hull. Also, as far to the center as possible before the angle up to the other side. There was an old similar style transducer there when I bought the boat. I am not sure, but from the looks of it, it was probably made in the 70's.
Although the location I used was textured from the fiberglass mat, it was not painted. I just sanded the surface flat and glued it down. I works really well.
I went with the "hawk Eye" brand. I think it was 69 dollars, and rated really well for my purposes. I reads really well, it has a green back-light, and has a correction function so you can get an actual water depth reading. I didn't use the correction, as it is more important to me to know how close I am to the actual keel, as I am with the actual depth.
Steve