Single-walled carbon nanotubes (SWCNTs) greatly enhance photoacoustic effects in aqueous solution of poly(ethylene glycol) via strong near-infrared sensitization and cavity formation. The observed acoustic attenuation and sound speed dispersion reveal polymer entanglement and laser-generated cavitation. Bubble resonance accelerates the sound wave and induces strong nonlinearity. As a versatile nanometer-sized in situ photoacoustic emitter and bubble generator, SWCNTs may find applications in microscopically controlled photoacoustic imaging and low-amplitude shock wave therapy.