Never mind. I believe I found an easy solution to this.

I initially set the original "standing" collision hull measurements before the while loop. Including creating a var called "inTheWater = 0"; and also initially set "water_mode = 0", since player starts out on land.

In the while loop, I created 2 conditions, "if ((water_mode == 0) && (inTheWater == 1))", and "if (water_mode == 1)". In the "water_mode == 1" condition (meaning the player is in the water) I would set the new "swimming" collision hull dimensions, and change the inTheWater value to be 1. In the "water_mode == 0 && inTheWater == 1" condition, I set the collision hull all to 0's, and created a goto function call going back to the beginning of the function. Now it seems to work.