low-med priority

• converting an area of biome to river biome doesnt really do anything at the moment. no drown, squids or salmon will spawn. the only real benifit of river biome on skyblock at the moment would be to stop all animal spawning if thats something a person wanted. 
• I tested this on my skyblock world just to be sure it wasnt just me. converted the area, flooded the area in a similar manner to making a river biome drown farm. absolutely nothing spawn in the water. if it wasnt dark enough for mobs then at least fish and squid would spawn, however they do not.
• hoping this will be fixed before skyblock is converted to 1.19

Just asking, did you relog after changing the biome?

Edit: Ignore me

yep yep. biome is correctly changed.