Seasoned snorkelers and sellers of fins would always hold back a chuckle when a "newbie" would ask if asked which fin was the left and which was the right. If one thinks about it though, the question is actually justified because the pair are sold by shoe size and shoes do have a left and a right.
The history can be dated back to 1718 when a 12 year old inventor by the name of Benjamin Franklin decided that making some out of wood would help him to swim faster but he made them for his hands which is the same concept. Obviously, made of wood were a bit impractical and it wasn't until about 1935 when rubber started to be used in the making which was introduced by a Frenchman by the name of Louis de Corlieu. He made incorporated metal bands and hard inflexible rubber and the flexibility of the blade entered the picture in 1940 and this was credited to Owen Churchill who began using vulcanized rubber in the creation of his fins. Surprisingly his idea came to him following his return from a trip to Tahiti and he saw the natives weaving small mats from palm fronds. When completed, the mats were dipped in tar, cooled, then tied to the feet before the natives entered the ocean to free dive for their meal. Read more on the History of Fins.
All of this while the fins were designed without there being a differentiation between a left and a right. In fact this uni-foot pattern continues to this day with 99.9 percent being manufactured. There is one exception that is known to this author at this writing and it is the Alpha which are sold by Aqua Sphere.
The Alpha Fins are actually designed for swim training and each pair is marked as being left and right specific. They go against convention by being made using a non-water absorbent foam material which makes them extremely lightweight and makes it great for traveling. Having originally been designed as a swim aid, they have also gained the notice of snorkelers as well. Their purpose is to help condition and strengthen the leg muscles used for kicks while swimming. With this in mind, those using them for snorkeling may find them to use a bit more energy for kicking than conventional snorkeling styles.
This to the knowledge of this writer is the first and only introduction that is actually has a right and left foot designation. So unless the style that you are purchasing are the Alpha, the answer to the question of which is the left and which is the right remains true with it just doesn't matter.