my guess is that your playing ocarina of time [which ironically im also playing right now i just got into gannons castle] . Well you could use the walktrough on here. But since I beat the water temple yesterday,there was one part in that temple i kept missing. If you go into the middle tower,and play Zelda's lullaby
(< ^ > < ^ > ) it will make the water inside the tower rise. It should have a small cut scene showing the water rising and a block platform rising with it. Go under the block and use your iron boots which will bring you to the bottom but that block was..blocking a hole you cldnt enter unless you made the water rise. oh and a word of advise get your hookshot ready. Hope I helped,if not then just use this sites guide.