Leylins Swimming Trunks
This magic item is helpful not only for being a step up from diamond, but also for travelling across large bodies of water, especially with Depth Strider.
It is enchanted with Protection V, and gives the user Water Breathing II, Dolphin's Grace II, and two extra hearts upon wearing it. It is also unbreakable.
Where to Get It
It can be found in Magic Chests.