No fish are spawning in any world, even brand new ones. The only fish in my worlds are ones that were spawned in before snapshot 19w13a. To test this, I created a fresh new world in 19w13b, flew to the nearby ocean, waited a few minutes to give ample time for fish to spawn, then ran the command /tp @e[type=minecraft:cod] <username>, which would teleport any cod loaded in to me. This command was unable to locate any cod. I have repeated this test in multiple locations, and with salmon over rivers, but no entity is ever found. Dolphins, squid, and turtles all appear to still spawn normally.