Inside water, the air bubble behaves like a Concave lens. The concave lenses are known as diverging lenses as the rays diverge after falling on the concave lens.
The air bubble is a sphere with a convex surface and when light reaches it, the light is reflected within the water as in a concave lens.
The refractive index of air = 1 and
The index of water = 1.33.
Since the water refractive index is greater than air, the air bubbles in the water will behave like a concave lens.