IP ratings are the foundation. IP66 survives splashes and spray. IP67 survives temporary immersion (1 meter, 30 minutes). IP68 is rated for continuous submersion but depth varies by manufacturer.
Gaskets matter more than people realize. EPDM outlasts generic rubber and survives UV. Silicone works but can flatten over time. Foam gaskets are budget-friendly but fail after 1-2 seasons.
Latches are the weakest point. Look for stainless steel hinges with spring-loaded over-center latches (Southco, Pelican style). Avoid plastic clips — they brittle and crack in cold.
For electronics: use pressure-equalization valves (Gore-Tex style). Without them, air expansion from temperature changes blows seals apart. Most reputable waterproof cases include them.
Pro tip: test new cases in a sink before trusting them on the water. Fill with paper towels, submerge for 5 minutes, and check for moisture.