Holes
I personally do not like any more holes in the hull than absolutely necessary. If you want a speed instrument, then a thru hull is necessary. If you don't find the speed instrument a necessity then you can do away with the holes completely. A depth transducer can be placed inside the hull, and will work just fine. I have a hawkeye depth in my boat. The transducer epoxied inside the hull. Works good, no danger of it getting knocked off, and no thru hull fitting. As for boat speed, I use a GPS, which gives speed over ground, but not speed through water, which I don't find necessary for me. You can tell if a inside transducer will work on your boat, by putting the transducer in a baggie of water and sitting it in the bottom of the hull. Move it around till it gets a good reading and then use a good epoxy to stick it down in that spot. I used Marine Tex on mine.