Brillouin scattering has applications ranging from signal processing^1,2, sensing³ and microscopy⁴ to quantum information⁵ and fundamental science^6,7. Most of these applications rely on the electrostrictive interaction between light and phonons^3,7,8. Here we show that in liquids optically induced surface deformations can provide an alternative and far stronger interaction. This allows the demonstration of ultralow-threshold Brillouin lasing and strong phonon-mediated optical coupling. This form of strong coupling is a key capability for Brillouin-reconfigurable optical switches and circuits^9,10, for photonic quantum interfaces¹¹ and to generate synthetic electromagnetic fields^12,13. While applicable to liquids quite generally, our demonstration uses superfluid helium. Configured as a Brillouin gyroscope¹⁴ this provides the prospect of measuring superfluid circulation with unprecedented precision, and exploring the rich physics of quantum fluid dynamics, from quantized vorticity to quantum turbulence^15,16.