Engine raw water cooling parts do require periodic FULL off engine servicing. Barnacle Buster will not fully meet the system's needs. For example: if your engine has an aftercooler, the air side of this "heat exchanger" will eventually clog up with oily deposits restricting air intake. BB will not do anything for this condition!
Also, the various components should be disassembled, cleaned, new seals or O rings installed, and then be pressure tested to ensure that there are no leaks from one side of the exchanger to the other. Corrosion can lead to leaks, and you don't want sea water in your oil, transmission fluid, fuel, coolant, or going in the air intake. Very expensive repairs or replacements could result. BB is really a temporary help to address part of the need, maybe a mid season use to allow for an extension of time to do a full service in the fall?
Depending on several factors like local salinity, growth, etc. this full service should occur every 4-6 years. See sbmar.com for more info from Tony Athens, a marine diesel guru (under Tony's tips or on his forums).