I have just found the way. Below are some info I gathered.
-Alight at Marina South MRT and go to Exit A
-Take Bus Number 402 (approximately one bus every 15mins)
-Alight at Marina South Pier
-Ferry departing to Kusu Island at 1000hrs
-Admission/ Return ticket: $11 altogether
-Ferry departing from Kusu to Singapore: 1145hrs, 1415hrs (will go to St John Island then to Singapore), 1515hrs (last ferry)
-No food and drinks are sold there.