Installing an underwater light in an in-ground pool without an existing niche is more complex that most DIY types will want to tackle, though it is possible. To install an underwater light you need access to both sides of the pool wall. Typically it is very difficult to get access to the outside of the pool wall on an in-ground pool, as it is often underneath the deck. There are also a variety of special electrical wiring rules that apply to underwater lights. Following all of them requires a good understanding of the version of the electrical code in your area. All of that can be figured out. Still, it is a significant project, requiring some time for research and a fair bit of digging, but it is not really difficult in the sense that it doesn't require any special skills that take a while to master.