Whenever you have dissimilar metals below water.. Your engines RW circuit is also filled with electrolyte and has numerous dissimilar metals...
You usually don't unless you are mixing brass and bronze, which MANY boaters and builders do.. Dangerous, but it happens all the time... This was taken in 2011 of a 2011 production boat:
This is what can happen when you mix 85-5-5-5 bronze with el-cheapo yellow brass Home Depot fittings. The yellow brass male adapter literally crumbled.
So long as your boat is electrically safe, as in stray DC current, and your seacocks are all of a similar metal, then bonding can certainly be optional. My own boat is not bonded but It has no DC or wiring issues and 85-5-5-5 bronze seacocks...
Sometimes those metals are isolated and not in contact with other metals other than through the electrolyte. They may also be a high quality 85-5-5-5 bronze... That said there are plenty of "isolated" prop struts that snap due to dezincification, usually because they were cheaply cast manganese bronze.
That's a big "if"... I have never seen a silicon bronze recreational prop or shaft most likely due to how machinable the material is or is not... Shafts are either SS, Aqualoy, Nitronic, manganese bronze or Tobin bronze and props are usually manganese bronze, SS or NiBrAl...
Most props, the majority, are Manganese Bronze which is upwards of 35-40% zinc. Older Tobin Bronze shafts were also upwards of 40% zinc. Slap a manganese prop on an AQ-22 shaft and you're going to need a zinc.. Now add one of the graphite packings and you will even more DEFINITELY need a zinc.... Today no one sells bronze shafts and they are SS or one of the Nitronic or Aqualoy variants. The most common saltwater shafting today is Aqualoy 22..
Often machining ability & casting ability and how it deals with pitch adjustments etc.. Manganese bronze is highly formable but they add some tin to it to help minimize dezincification...
Most good quality bronze seacocks, Apollo/Conbraco, Buck Algonquin, Spartan, Groco etc. etc. are cast from 85-5-5-5 bronze which is very, very, very resistant to dezincification and far better than a manganese prop or shaft.. There is a big misconception that these are silicon bronze when in-fact they are 85-5-5-5 bronze (copper, lead, tin, zinc).
Some rudder shoes etc. are made of this too and some are actually made of silicon bronze.. Cheaper builders through still use manganese bronze for p-struts and rudder stuff and they can and do fail due to dezincification...
This is what graphite packing can do locally to a bronze shaft even with an intact zinc, though poorly installed, at the other end of the shaft..... It took less than a season to destroy this bronze shaft when graphite was added to the mix. The damage was localized to the area where the packing made contact with the shaft...
Keep in mind that the electrical potential of graphite is +0.20V to +0.30V and manganese bronze is -0.27V to -0.34V. This creates a HUGE corrosion spread!!!! The further away your metals are in the galvanic scale the more need for zincs....