Would depend on where it's leaking from. If it's the housing or a gasket or hose connection. There is probably a gasket or o-ring type seal on the impellor cover. Other than that unless it's leaking at a hose connection the housing might have a crack.
Your boat probably has a March magnetic impeller pump. They are designed to move water, not build pressure. The business end of the pump is a plastic housing. If the leak just began to occur, then it is very likely that the cooling lines are fouled and the system needs to be cleaned. If this is the case, the leak will occur along t he gasket that seals the impeller housing from the flywheel housingnear the end of the pump. YOu wouldn't replace the pump just because the lines are fouled, however, given the age of the boat, it is about as likely that the pump housing is just cracked and the pump needs to be replaced.