According to the Countries of the World by Borders in 90 Seconds Quiz, Sri Lanka has a border with India at low tide. But you're right about Singapore. I've removed it. This quiz will now have a very strange URL.
But they dont, I researched it. They have a bridge but I wouldnt call that a border, and there used to be a complete landbridge that was passable by foot, but that was over 5 centuries ago. Now only parts of that landbridge are above water (16 islets) and the rest is under water, up to a depth of 10 meter.
According to Wikipedia the sea is only one metre deep and this is "rarely" exceeded. This means that in some events (I think a tsunami was one?) the bridge is passable by foot. I wouldn't necessarily count this myself but it is the policy of Jetpunk.