The marine industry, like the automotive industry, has a substantial aftermarket parts industry. Unlike the automotive parts industry however, many are owned by the OEM manufacturers themselves. In comparing oem to aftermarket parts, the distinction lies in the fact that oem parts are updated more often and therefore are better for your engine. In the sub categories of aftermarket parts, electrical is one area to avoid aftermarket, the quality is never as good as oem. In gaskets, some aftermarket parts are actually better. Carbuerator kits are better as oem, while gearsets and seals are equal.
The best way to look at choosing parts is warranty. OEM dealers will stand behind parts that are found to be defective if they installed oem parts but not if they installed aftermarket.