hey hey Judicator
Keep in mind that there are always limitations in games, if any game, no matter how tripple A or tripple S they are, if you constantly press pause eventually the player will see things that will look unrealistic.
Heck, if anything most aspects of game development are smoke and mirrors anyways.
If you want to reduce the fin being noticeable try adding one or two more, change the opacity, or use other blending modes (alpha composite, additive, etc)
Additionally some sprites/sparks moving down the fin can also help.
you could try the drop-shaped mesh, but besides using more polygons, if you are using the regular rendering system the limitations of translucency might result in it looking wonkey to say the least.