You should place your codes inside a while-loop, otherwise
it never call swim_mode() even is_swim_mode == 1.

Code:

---

action	PlBiped01
{
  while(my != NULL)
  {
     if (is_swim_mode == 1)
     {
        swim_mode();
     } else {
       // the rest of PlBiped01 action code
     }
     wait(1);
   }
}

---