Minecraft wiki entry for Amethyst_farming, associated text: Some water buckets
Some conduits and enough prismarine to make all of max-level
Some blocks or glass
If you are building it in an ocean biome, some light sources that work well underwater, such as sea lanterns, glowstone or jack-o-lanterns are required. This is needed to prevent drowned from spawning. If you are not building this in an ocean biome, you will not need them, since you can already see clearly underwater with the conduit power.
Optionally at least 1 soul sand and 1 magma block for a bubble elevator
Optionally a Haste 2 beacon if you are digging out a large area