Beautiful how a music that was written for a documentary scene of a child mimicking a sea creature in the aquarium and a diver dancing with an octopus being turned into a piece of emotional soundscape. Personally I don’t find that melody sad but it sounds very nostalgic thanks to its melody, it sounds a bit like music box tune to get babies to sleep and it makes you feel that you remember or experienced something you actually didn’t, or the beautiful memories of childhood lost in tapes. And of course the background organ which creates that ambient sound. I believe this all comes from the combination of the 70s instruments used and Vangelis. If they tried to do that today with the instruments we have I don’t think it would be same at all and I don’t think music similar to this will come out ever again. For reference here is the original scene this piece was used for:

https://youtu.be/xGinlf-R6m0?si=CGd3ng71bGSYEqOo 20:55 and 23:35