When evaluating safety whistles, the primary metric is decibel (dB) output, which should ideally range between 100dB and 120dB to ensure audibility over ambient noise like wind or machinery. You should prioritize pealess designs (no internal moving parts) because they are more reliable in extreme conditions, as they cannot freeze, clog, or fail when wet. Additionally, ensure the material is high-impact ABS plastic or corrosion-resistant stainless steel for durability in marine or outdoor environments.