reatedb: database creation failed on mydb

Hi!

I'm a new member and new to postgres.

beside postgrest user I would also want my student to create their own
database in their own home directory.

first I issue (under postgrest user)
    createuser user_id
then under that user_id I run
    initlocation ~/sql/data
then I run
    createdb -D ~/sql/data mydb

at this stage I get an error msg as below:
    ERROR:  Unable to create database directory ~/sql/data/base/mydb
    createdb: database creation failed on mydb.

what should I do now? I tried to find it in archives but i can't find it.
FYI I'm using postgres v 6.3 running under RedHat.

Thanks in advance.