Create a default BuoyancyInput. The PhysicsBody.BuoyancyInput.mask defaults to PhysicsMask.All so every attached shape contributes unless explicitly filtered out. The surface defaults to a flat horizontal water surface at the world origin (PhysicsBody.BuoyancyInput.surfacePosition = Vector2.zero, PhysicsBody.BuoyancyInput.surfaceNormal = Vector2.up).