EntityPrehistoricSwimming such as Icthyosaurus will only spawn on land when use BiomeTweaker to make it spawn as map generating.
code of BiomeTweaker scripts:
jurassicBiomes.addSpawn("fossilsarcheology.server.entity.prehistoric.EntityIcthyosaurus", "WATER_CREATURE",5,2,4)

Beside, EntityFishBase such as EntitySturgeon works well.