Spawn rate of ocean mobs is too low.

After updating a world to 1.13, very few ocean mobs spawn. Squids used to spawn very frequently, but now it can take several minutes and many oceans to find any. Turtles and fish are very scarce too. Drowned zombies seem to be the only mob spawned with any consistency. The spawn rate seems to be unaffected by the age of the ocean. Pre-1.13 generated oceans as well as the new oceans lack mobs. In a new world created in 1.13 there were plenty of spawns within seconds.