In situ monitoring for complex multiphysical field changes of underwater environment has urgent need for the miniaturized integrated multifunctional sensor. We have proposed and demonstrated a compact three-wavelength fiber laser sensor designed for simultaneous measurement of underwater pH, salinity, temperature, and axial strain, emphasizing miniaturization and integration. The entire sensing probe consists of a 75 mm length, comprising a 5-mm single-mode fiber (SMF)–multimode fiber (MMF)–hollow core fiber (HCF)–MMF–SMF (SMHMS) interferometer cascading a polarization-maintaining fiber Bragg gratings (PMFBG) and an etched FBG. The surface of the SMHMS interferometer has been coated with silane coupling agent-polyvinyl alcohol/polyacrylic acid (SCA-PVA/PAA) coating, while the etched FBG is modified with a coating of SCA-polyimide (SCA-PI). By monitoring three laser wavelengths and output powers, the proposed sensor enables the simultaneous measurement of underwater pH, salinity, temperature, and axial strain. In our underwater experiment, the intensity of $I_{\mathrm {FBG}}$ exhibits a sensitivity of 2.577 dB/pH during pH rising process and 1.930 dB/pH during pH falling process within the pH range of 7.4–8.6. Additionally, the wavelength responses of temperature are 0.040, 0.009, and 0.0098 nm/°C, respectively. The characteristics of salinity are obtained with sensitivities of 0.195 dB/wt% and 0.10 nm/wt%. Furthermore, the axial strain sensitivity is determined to be 7.6 $\times \,\,10^{-4}$ nm/ $\mu \varepsilon$ and $-7\,\,\times \,\,10^{-4}$ dB/ $\mu \varepsilon$ , indicating the wavelength and intensity sensitivity to axial strain. The favorable repeatability, stability, and linearity of the fiber laser sensor have been experimentally demonstrated.