Despite their lack of moving parts, heat exchangers still need regular attention. The seawater side is, of course, exposed to a variety of contaminants, ranging from salt, depleted zincs, and impeller parts to hard and soft marine life, sand, and silt. The coolant or closed side of the heat exchanger, however, can also suffer from blockages that are a result of exposure to old, contaminated, or acidic coolant; most engine manufacturers recommend that coolant be replaced every two years. Corrosion can also take its toll, leading to either external leaks or leakage between the raw and closed cooling circuits, which often forces coolant into the seawater.
The construction of heat exchangers can also conspire against them. Exchangers can be manufactured from more than one copper alloy, and this can lead to galvanic corrosion. It's for this reason that most use a sacrificial anode or zinc pencil, although it's of little value unless it's replaced at regular intervals. The frequency varies from engine to engine and boat to boat, so check yours at least monthly.