Full-spectrum underwater optical communication (UOC) is of great significance for major strategic needs including resource development, scientific exploration, and homeland security. As the core of the full-spectrum UOC system, photodetectors (PDs) are plagued by stringent requirements including a broadband response, intrinsic water resistance, and a high detectivity. In this work, two-dimensional (2D) halide perovskites (HPs) and corresponding PDs are constructed by stearamine (SA), representing the rarely explored long-chain aliphatic amine series, to own waterproofness, ultralow noise, and superior optoelectronic performance, which consequently enable a high suitability for UOC. By dimensionality and composition modulations to extend the absorption onset down to 1.5 eV, a broadband response covering the entire transmission window of water (> 1.55 eV) for full- spectrum UOC can be obtained. Besides, featuring a high responsivity of 3.27 A·W⁻¹, a peak external quantum efficiency (EQE) of 630%, fast rise/decay times of 0.35 ms/0.54 ms, a superior detectivity up to 1.35 × 10¹² Jones and the capability to distinguish various waveforms and light intensities, the PDs present sensitive and persistent photoresponse underwater. As a result, proof-of-concept wireless transmission of ASCII codes in water is demonstrated.