Some said to use epoxy for permanency, but what if you ever want to remove it? I've mounted several with clear silicone, and while it's horrible, messy stuff, it works very well for this purpose. Use the caulking gun size, not the toothpaste tube. It's hard to get enough of of the tube without getting bubbles in it.
You'll lose a little top end range going through the hull, but unless you're running bottom surveys that shouldn't matter. (I run a research vessel, and it does matter there).
Best location would be forward of the keel, worst would be next to the keel, it might not work well on one tack with the keel in the way.