Like the title says I believe it’s tidal cores I need, or maybe it’s hydrocores. I forget which is which. Anyway it seems that whenever I join a heroic dungeon, none are getting dropped. I can’t find anyone running mythic ones either. I’m a noob so I can’t exactly get into mythic plus. My item level is 422 btw. I’d really like some farming tips on how to get them as I want to start making embroided deep sea bags and selling them. TIA!
I think Tidalcores are the ones that drop. You mainly get them from Mythic dungeons but they can drop in Heroic. Hydrocores can only be gotten from using the recipe given to each crafting profession that converts a Tidalcore to a Hydrocore.
The Embroidered Deep Sea Bags use Hydrocores so you need to get Tidalcores and use “Aqueous Alteration” to turn them into Hydrocores for the recipe.
I keep getting confused which is which xD. I know one gets converted to the other but always forget which one it is 
Maganar is correct about the switch from tidalcore to hydrocore. My understanding and experience of tidalcores is that there is a 100% chance for one to drop from a mythic or mythic+ dungeon, and a limited chance; maybe 20% or so for it to drop from a heroic dungeon. Your best bet is to try to find a group for all of the mythic(430) dungeons for you weekly lockout and then as you get some better gear you can just farm +2/3s for the speediest way to get a guaranteed tidal core drop. Good luck!