Hello. These are called vitreous floaters and is generally due to a process of degeneration.As the vitreous degenerates small strand or wave like clumps form inside it and it reflects onto the retina especially in bright light against a clear background (like the sun or the swimming pool). As you have not mentioned your age, the cause of this floater cannot be postulated.However ageing,short sight(myopia),trauma,diabetes,inflammation,etc are few causes. Please get an dilated eye examination from an eye specialist,who can tell you if there are any retinal tears present.If the retina is fine,I would not worry too much.