Hiding your eyes, isn't necessary, but avoiding turning "head on" and focusing on a fish is necessary. Every fish grows up eating or being eaten by bigger fish. And fish don't have a neck, so fear another fish that turns "head on" and looks at them. A diver can turn his head, and avoid pointing their body at the prey. Your whole body position can scare or be ignored by fish.
In choosing a speargun, the first consideration is visibility where you are diving. I dive in fresh water lakes with visibility from 2 feet to 10 feet most of the time and 20 to 30 feet at times. Using the JBL dimensions, their 38 special works well out to 10 feet. With the two bands, If you get the 38 special NW it comes with 3 notches for added power and will allow bigger fish or a couple of feet more reach. If your diving area has 10 to 20 foot visibility you should look at longer guns.
Prices on spearguns will vary by how many guns the manufacturer turns out (automation) and how much hand work is involved. Manufactured guns put a lot of fish on the table. . . Custom guns can get the fish that can "get away" from lesser guns. You can have a lot of fun with the inexpensive guns, and a lot of fun with and pride of ownership with a more expensive gun.
I started spearfishing with pneumatics which are fast reloading. I've had and used 4 different guns, when the manufacturers quit making seals, they were worthless. I got a "woody" 38 special with a 3 notch shaft, changed the bands every year, and I still have it. I wanted better, and for more money got a riffe metal tech. It cost more but was faster, more powerful and turns quicker because of the mid handle.
There are a lot of variables that affect accuracy. But a straight shaft, powered correctly, in a balanced gun, can hit what you aim at. Aiming a speargun does not mean looking down the gun, you can ge hit in the face. Practice shooting the gun to always hold the grip the same way and practice shooting small targets, I shoot a lot of beer cans littering the bottom. You will become familiar with the grip. . . If you are shooting high. . . concentrate on the bottom of the target.
When you have trained your eye/grip you're set to go until you pick up a gun with a different grip, then start over.