A technician notices air bubbles visible through the clear section of a raw-water sea strainer. What is the most likely cause?

The presence of air bubbles visible in the clear section of a raw-water sea strainer is indicative of a leak that allows air to be drawn into the water intake system. Among the provided options, a leaking gasket in the sea strainer is the most plausible cause for this observation.

When the gasket seals the strainer are compromised, it creates a pathway for air to enter the system, which can result in the formation of bubbles as water is pulled into the strainer. This occurrence disrupts the intended flow of water and may lead to reduced efficiency in the cooling system, as well as potential overheating if the pump fails to circulate water properly.

Other potential sources of air entry, like a leak in the raw water pump, are less visible due to the pump's operation further downstream in the system. Similarly, a blockage in the compressor discharge would not typically cause visible bubbles in the strainer but rather would lead to a variety of cooling performance issues elsewhere in the system. Lastly, while excessive heel in a boat may affect water intake, it is not directly responsible for the presence of bubbles within the strainer itself.

Thus, a leaking gasket in the sea strainer is the most direct explanation for the bubbles observed.