When affected by dolphin's grace, it basically works the same way as depth strider, which is intended to make you travel across the water faster (as Jeb has said a while ago). Dolphin's grace, unlike depth strider, is intended to make you actually swim faster in any direction, so that you can dive faster. However, this is not the case, and Dolphin's grace turns out to be annoying when trying to dive (because I move too fast horizontally), and it is also no different that depth strider.