A tomahawk is a TOOL. If you use that tool, it will develope wear, dings, dull the edge, and the handle will loosen up. It just happens when you use a tool hard. And if you throw it, that puts a lot of hard SHOCK into the junction of the wood handle and the metal head. That "shock" will loosen up any handle over time. It just happens.
The handles on my blacksmithing hammers loosen up all the time. It's just a fact of life. Like any well-used tool, it needs occasional repair and/or tweaking.
Soaking your hawk (or hammer) in water will tighten it up at that moment. It swells up the wood. But it also crushes some of the wood cell structure. So when it dries out and the wood shrinks, it will be looser than before. Then next time you soak it in water to tighten it back up, it will not swell up as much as before. And it will be a little looser when it dries back out. Something of a vicious circle.
So if you throw your hawk, just expect the handle to loosen up occasionally, and possibly break. It's a known condition. You just have to learn to deal with it.
And most people are better served by buying one of the lower priced hawks out there. They will do most of what people require of them. Once you have some experience using a hawk, then is the time to search out one that may better fit your needs.
Just my humble thoughts to share. Take them as such.
Mikey - yee ol' grumpy German blacksmith out in the Hinterlands