When I first started spearfishing I never used to carry spares around. Part of the reason for that is because I live in a main city and spares are easy to get. In saying this, I did do a two week trip up north where I wouldn’t have been able to get spares and I didn’t even take a spare set of mono line or rubbers! What I didn’t realize at the time was how easy it is to break something, whether it is the main line, a rubber or simply to bend or lose a spear! https://inspares.in/
If you aren’t able to get supplies quickly and easily, you need to take at least a spare spear, plenty of mono line, crimps and the metal crimps, pliers, a spare rubber and a spare bungee cord. Having a spare bridle is often handy in case one pops out of the rubber. These spares hardly cost anything (the spear is usually a bit more though) and when you are away from town for a few weeks they are well worth it. Lucky for me nothing broke on the gun, but if it did I would have had no spearfishing for a whole two weeks, which was the whole reason I went there in the first place!
If you are a serious about spearfishing carrying two spears, a spare firing pin and trigger setup along with spare float line and floats will usually be next on the list. At the end of the day spearfishing items are rarely replaced, but having a spare with you gives a good peace of mind and enables you to have a hassle free trip. Some people even carry a spare gun with them, which comes in handy too!