Another way:
x=$'Some\nstring'
readarray -t y <<<"$x"
Or, if you don't have bash 4, the bash 3.2 equivalent:
IFS=$'\n' read -rd '' -a y <<<"$x"
You can also do it the way you were initially trying to use:
y=(${x//$'\n'/ })
This, however, will not function correctly if your string already contains spaces, such as 'line 1\nline 2'. To make it work, you need to restrict the word separator before parsing it:
IFS=$'\n' y=(${x//$'\n'/ })
...and then, since you are changing the separator, you don't need to convert the \n to space anymore, so you can simplify it to:
IFS=$'\n' y=($x)
This approach will function unless $x contains a matching globbing pattern (such as "*") - in which case it will be replaced by the matched file name(s). The read/readarray methods require newer bash versions, but work in all cases.