i am trying to make a bone serpent from alexs mobs spawn in a deep ocean biome but it wont work, everything is in the spawner.json, i am on the latest version of the mod for 1.20.1 forge. when i try to load i get "Invalid condition 'biome' for spawner rule"
[
{
"mob": "alexsmobs:bone_serpent",
"persecond": 0.02,
"attempts": 10,
"amount": { "minimum": 1, "maximum": 3 },
"conditions": {
"dimension": "minecraft:overworld",
"biome": "minecraft:deep_ocean",
"maxheight": 126
}
}
]

You cannot use "biome" in spawner.json like that. It has to go in the 'and' block. See wiki for info on that
Also you are forgetting to use "inwater": true in your conditions block. That's required to enable spawning in water

also any way to make mods spawn at negative coordinates? I can't seem to find anything on the wiki for that.

Check the spawner.json examples. There is a 'minheight'/'maxheight' there