Having the ichtysaur was the original intention , but they decided on using the leeches instead , and I understand why . An ichtysaur has to be spawned in relatively close to the player every time he goes too far out and find him fast , and expecting the player to believe that the oceans could support an ecosystem with one of those in every ten cubic meters of water is preposterous . The leeches are easier to explain , and they ' re also easier from a programming perspective . Their small size allows them to be treated more like a particle effect than an actual creature , spawning in in front of your face , attacking you and then disappearing . Small size and large number means they can do this , whereas an ichtysaur would need to be a fullfledged npc with movement and pathfinding and shit , and it would probably not work too well in more shallow waters . Besides , you off a few ichtysaurs in HL 1 , so it ' d be strange if you couldn ' t just straight up kill them in HL 2 . As tools to prevent the player from reaching the skybox , I award the leeches my coveted Better than an Invisible Wall award , and hope that it doesn ' t go to their heads .