Let me suggest...
Take MRT from Sembawang
Alight at Toa Payoh MRT, Take 31 from Toa Payoh Interchange.
(This should be the fastest).
For 31, alight at Opposite Parkway Parade Bus Stop and cross over
the overhead bridge.
OR
Alight at Marsiling MRT, Take 966 from the opposite side of the MRT
station.
For 966, alight at the OCBC Bank Marine Parade, which is the
Parkway Parade side.
For both options, walk across the HDB blocks. You will see a
standalone NTUC supermarket. Walk beside the HDB block and the NTUC
and you will see an underpass to East Coast Park.