for flat water kayaking: start at Stadium Road, near Singapore Dragon Boat Association (behind Mardeka Bridge, follow to Kallang Reservoir, follow to the mouth of the River(s), to Marina Bay (as far as not chased by PUB people), back along the shores to the mouth of the River(s) - to the Sea almost, back to Kallang Basin - this one lap is more than 10km, do several and you are done.
That's the longest, for sure, except open Sea waters.
See you there!
Marek