Method: Likely the trapped air does help buoyancy, but the walking part is pretty simple: to tiny creatures, that surface is like a solid or semisolid plane (depending on their weight). Think of it as an upside-down water strider.
* Lurk in wait for prey hitting the surface. (This assumes it is predatory like most water beetles.)
* Avoid predators from above. It is hard to see under the water, making cross-planar predation less common.