When installing a reverse-cycle AC unit below the waterline, the best option for the condensation drain is:

Using a "shower-sump" to pump overboard is the best option for the condensation drain when installing a reverse-cycle AC unit below the waterline. This method ensures that the condensation, which can amount to a significant volume of water, is effectively removed from the unit without risking flooding the bilge.

Employing a shower-sump system allows the condensation to be collected and pumped overboard, which keeps the interior of the vessel dry and prevents potential issues related to water accumulation. This is especially crucial in marine environments, where excess water in the bilge can lead to a host of problems such as mold growth, equipment corrosion, or even vessel stability issues.

In contrast, draining directly into the bilge could lead to water pooling, creating an unsanitary environment and potentially disabling bilge pumps. Draining near a bilge pump might still retain some water in the bilge unless the pump is activated frequently, which is not an ideal scenario. Routing the drain to exit above the waterline, while effective at preventing backflow, can complicate the installation and may not be compatible with the operational needs of the AC system, especially if the unit is consistently generating condensation.

Therefore, the use of a shower-sump system provides the