I prefer interior due to the sensor is less prone to becoming damaged. Interior can be less reliable for accuracy and may stop working in some cases, depending on type of transducer and mounting location, more variables involved.
If you wish to mount externally, the reliability and resolution are usually better to some degree, but the sensor will be vulnerable.
I find following manufacturers directions usually produces acceptable results but be prepared in case the system fails to work properly in all cases. For instance, I suggest performing some in-water testing before drilling a new hole(s) in the helm for mounting the control panel.
Sorry for the very late response, I installed it in a Seadoo sportster so it will be different than a ski. I would presume the back of the ski would be the best spot. It is a big game of trial and error. It took me hour and hours to find the spot.
Next time for quicker response, quote my post or something so it notifies me.
Using a Garmin GT15-IH (in hull) mounted in starboard rear of 2017 GTX 155 with Garmin 43cv Chirp. My ski has a lot of area for installation in the starboard rear, and no foam core. Deadrise about 25 degrees.
The transducer is not mounted as far aft as I would like it, but it's extremely difficult to get to the transom area inside the hull (unless you're a contortionist). So, it's not too far behind a step in the hull, which is not good as the step generates turbulence at speed.
Still, I hold a bottom up to 30 mph and I'm happy with that. I'll try to remember to take a photo in the near future and post it.
My experience in finding info on-line, and in manuals for previous year skis for locating the transducer in hull is that there's a lot of different configurations for the different skis. Without specific models and years it is often confusing. (My friend's 2017 GTI has foam core in the area I mounted mine,
so no go without making a hole in the foam)
Mine was the type shaped like a little triangular wedge with rounded edges. It's meant to hang off the boat, but I managed to install it on my ski inside. Used marine epoxy. You must make sure there are no bubbles in it, or it won't read. I couldn't find a flat spot to install and realize that the deadrise gives me a slightly incorrect reading, but it's close enough. If I see that I'm in 3' of water, I start being very cautious. On my 97 Challenger, I installed it on the transom, and it works just fine back there. The Challenger had one there before, and I just used the same holes. Didn't want to put new holes on the ski.