To protect your marine grade boat subwoofers against water damage, it's important to ensure that they are installed in a location that is protected from frequent water exposure as much as possible. Even though they're designed to be water resistant, decreasing their exposure to sun and water will increase their longevity. Additionally, you can use protective covers or grilles to shield the subwoofer from impacts and direct exposure to water, and ensure that any wiring or connectors are properly sealed.