I have read and understand the principle of aiming down the side of a eurogun for better accuracy in the vertical plane where the fish is in it's smallest dimension.
But whilst a speargun has no sights front or rear its perfectly flat over the top. To me its the simplest thing to aim down the v created by the loaded rubbers and see the muzzle enclosure against the fish. I usually first place the muzzle / speartip on the fish and then lift the handle till i see the fish and the muzzle enclosure through the v of the rubbers. I shoot and he usually gets snacked that night.
Perhaps cos I grew up with a pellet gun welded to my hands and and mouthful of pellets and these days i hunt and clay and wingshoot plenty and a gun of any description in my hands feels more familiar than even my peeing tackle especially when i'm looking over the top of it.
Skin.