Water Hyssop

Bacopa monnieri, or Water Hyssop, is a low creeping perennial of wet ground found through the tropics on every continent. It runs across saturated soil and shallow water on fleshy stems that root at every node, forming a bright green mat only a hand's breadth deep and studded with small white to pale mauve flowers.
It is specified for the wet margin, where planting has to hold a bank, cover a mud line and tolerate periodic submersion.
Water Hyssop is the plant to reach for where the ground is too wet for anything else to hold.