You are correct, I had to cut a pretty decent size hole in the transom to allow for the transducer cable end (double split end for side view and down view) and fish that up to the dash, just make sure there is no plumbing or exhaust you may drill into. I used the Garmin mount and was able to place it nicely on the dash as I didn't recess it. I used the power from my courtesy lights switch to power up the unit as I never use the courtesy lights in the boat, or use the boat at night that would need inside lights.
If you plan to drill into the hull, put a piece of blue painters tape over the area you are going to drill into to prevent the gel coat / fiberglass from cracking. Start with a small drill bit and work your way up. Remember you are drilling into a boat, so the higher you can get the hole above the waterline, the less chance of water getting into the boat. I ran my cable as high as I could and that happened to be just under the radii of the swim platform. My '22 has pretty good transom access from the swim platform hatch by the clean out plug as I could measure, verify, mark, verify and start drilling without hitting anything with the bit.
Don't rush it, and wait to open that first beer once you are done drilling
It will be much more rewarding. BTW, what unit / brand are you looking to put in?
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