At this point I´m able to make a new model, that starts in standframe on land as well as when it´s in water.
As soon as the player is in view it start its attack.
On land it starts walking and follows the player into the water and starts swimming.
In water it starts swimming and shoots harpoons.
The only thing I am not sure about is :
how do I get a Quake1 monster out of the water on land?
As it seems code technical impossibe, since the engine has no support for it.