People can become beings who can travel through time by inserting a temoral segment of themselves into a kind of suit which allows them to "swim" "above" the surface of the "sea", where the sea represents time as we experience it. The time travelers can manipulate the time-bound world beneath them by moving and creating waves, like shaping the sand on the bottom of a lake.
Time is meaningless for five dimensional beings, but because they originate in our four dimensional world they perceive their existence as if time passes for them. When they've perceived as much time as the segment of their life they cut out in order to enter the fifth dimension, they "reset".
Some time travelers are basically five-dimension "natives" because they were "cut out" of their temporal lives by only a minute or a few seconds, so they perceive their world more closely to how it really is, without warping it through the lense of their experience of life as four dimensional beings. The beings are called Drums.
Others are very "long" - an especially powerful being is a 100 year long woman who was extracted from her life in order to take advantage of her extremely long reset time and this her strategic capabilities. She has to live in the"deep" part of the "ocean" - advanced along the time scale.
I read this book about 20 years ago, loved it, and have longed for it ever since. Who knows it's title?