Why is my array declaration incorrect?

Question

I'm trying to do some shizzle with SFTP and SSH but it would seem I'm getting hung up on super-simple array declaration.

According to this, I should be able to use the following syntaxes to create an array:

[bob in ~] ARRAY=(one two three)

[bob in ~] echo ${ARRAY[*]}
one two three

[bob in ~] echo $ARRAY[*]
one[*]

[bob in ~] echo ${ARRAY[2]}
three

[bob in ~] ARRAY[3]=four

[bob in ~] echo ${ARRAY[*]}
one two three four

The part from that for me to focus on is this line:

[bob in ~] ARRAY=(one two three)

I tried replicating in my script:

#!/bin/bash
USERS=("user1" "user2" "user3")
PASSES=("pass1" "pass2" "pass3")

for i in ${!USERS[@]}; do
    echo ${PASSES[i]};
done

Running this (using sh script.sh) outputs:

script.sh: 2: script.sh: Syntax error: "(" unexpected

Why is my array declaration incorrect? Is the guide just wrong?

I know I can use:

declare -a ARRAY

and then append to it, but I like the idea of using a one-liner wherever possible.

Thanks,