Reflection probes are one way. Basically a camera drawing a simpler version of the scene from a point into a cubemap. Decent for oddly shaped objects, although if you want a lot of them then you’d bake them and lose any real time changes. A common optimisation is to update them less than once a frame.

If you have one big flat plane like the sea, you can draw the world from underneath and just use that. GTA V does that (like ten years ago without RT), along with the mirrors inside. You could make that look better by rendering them in higher resolution.

adriancourreges.com/…/gta-v-graphics-study-part-2…

Where RT is visibly better is with large odd shaped objects, or enormous amounts of them. I can’t say it’s worth the framerate hit if it takes you below 60fps though.