A Surface Marker Buoy (SMB) is essential for warning other water users that you are present. Without a marker buoy, you are almost invisible to boat traffic.
Buoys range in price and style from basic and cheap through to more complex designs that have more to offer than simply marking your position.
Some marker buoys are of a single skin design, these are the cheaper option, as the single skin construction is less resistant to punctures than that of a double skin design. The double skin buoy consists of an inner bladder and an outer cover. The outer cover is made from a strong puncture-resistant material. More expensive marker buoys often have pockets for storing spares or snacks, as well as having loops and straps to attach your speargun(s).