There are lots of ways you can better prepare for a safe and successful spearfishing trip.
Safety should be your number one priority. Carry a whistle and mirror on your float. Even consider carrying water, energy bars, flares, GPS/EPIRB, knife, if practical. Always ensure you have a dive flag and your float is high visibility.
A rigid code flag alpha is preferred. In conditions with no wind, soft unsupported flags may not be visible. The longer the flag pole the better for superior visibility.
Let someone know where you are going and your expected return time. Try to always stay visible to your dive buddy.
When it come to spearfishing success, you should roughly know the target species you will encounter for a given dive and location. This enables you to select the most appropriate equipment for the task and be best prepared.
Carry and be familiar with the current regulations for bag limits and sizes in the area you are diving as well as marine parks laws and zoning.
Carry a soft plastic ruler for measuring.Have the equipment to clip fins or punch crayfish tails if necessary.
Only shoot what you can eat. Unnecessary killing or wasting of fish can be seen to give the sport of spear fishing a bad name.