Getting Water Lurkers to Work Properly...

[HakePT]

As far as I know you can set entities and enemies to non-active.
When you create them just check the box so they aren't active. This means they don't actually exist.

By checking the original maps by frictional games i found out that they use this to make maps change as player progresses or enemies appear when you do something. For example the black hall that changes when you have all the pieces to proceed.

These are all entities that aren't active until something happens.
Example: create an area (AreaX), the enemy (water_lurker_1) but not active and in the scrip say that when player collides with AreaX then water_lurker_1 becomes active.
This will make it so when the player goes into a certain area the water lurker spawns. Handy to make sure the player doesn't hear the enemy before you want to.

As to him not following you into land i'm not sure. You can try not setting a path for him there or just create a block area that prevents him from passing it. Still this is just me making theories on what would work.

PS: enemies can also be set as to ignore the player (you can dance in front of them and they still won't attack you), or to set them as hallucinations. Hallucination enemies act like normal enemies, chase you and so on but disappear when they get to a certain distance to the player.
Example: I'm planing on making a lvl where you go down a corridor open a door and a grunt appears just behind. You then run all the way down the path (no place to hide) till you reach a locked door. But the grunt is a hallucination so when he is just about to catch the player he will disappear. Still trying to find a way to make him drop a key when he disappears.