Abstract: The detection of weak underwater targets is a critical requirement for national underwater security and resource exploration, encapsulated in the "Two-One-Thousand" Problem: the target signal intensity is merely one-thousandth of that of interference sources, and targets are extremely sparse, necessitating highly reliable detection and identification in complex marine environments. Traditionally, the approach has relied on a "hardware confrontation" paradigm, enhancing performance through enlarging apertures, increasing power, and lowering frequencies. However, a "performance reversal" phenomenon is frequently observed in practice: when hardware capabilities exceed a critical threshold, overall system effectiveness declines instead of improving. This paper introduces for the first time the "Sonar First Paradox"—the "Detection Gain Reversal Paradox," also termed the "Dimensionality Disaster." This paradox reveals that in interference-dominated real-world environments, extending detection range synchronously triggers a polynomial explosion in the number of interference sources (surface interference ∝ R², volume interference ∝ R³), which far outpaces the exponential decay of the target signal, ultimately leading to a collapse in the signal-to-interference ratio (SIR) and severe localization ambiguity. By developing a comprehensive signal-to-interference-plus-noise ratio (SINR) model, this paper rigorously demonstrates the "unsolvability" of the two conventional paths—enlarging aperture and lowering frequency—showing that their marginal benefits approach zero under long-range conditions. Consequently, the paper argues for a paradigm shift from "environment confrontation" to "environmental collaborative cognition" and proposes a solution framework based on a five-dimensional collaborative perception system encompassing "bearing-range-depth-motion-feature." This study provides a new theoretical framework for understanding the performance boundaries of sonar systems and lays the groundwork for next-generation intelligent underwater acoustic detection technologies.