A spearo's float has three main functions:
1. To transport fish and spare kit
2. To make a diver more visible
3. To stop large fish
The best size for New Zealand conditions would be 10ltr. This has more than enough lift to stop 99% of the fish we shoot here and, more importantly, enough lift to hold 10 butterfish and a kingy without sinking!
For larger (30kg+) fish a bigger float is needed to stop them. A float around 35lt is normal for gamefishing.
The importance of a float for visibility cannot be stressed enough especially when diving offshore. Choose a brightly coloured one with flag.
Sausage shapes are the best as they have the least drag and, with the addition of a lead keel, will remain upright with a flag mounted on them. Flat ones are hopeless if you want to put a flag on them.
Foam filled floats will maintain their buoyancy even when pulled under and will eventually pop up somewhere whereas a hollow float will implode and sink.
Inflatable floats are very useful when travelling and quality ones are usually very durable.