Picture a flat board pushed across water faster and faster. At slow speed it sinks in and has to shove the water aside. Past a certain speed the water stops letting it in and starts holding it up, so the board rides along the top and barely touches anything. Almost everything this word means can be held in that one picture: the flat shape that water lifts, and the moment something climbs up onto the surface and rides there.
From the word’s history
Wiktionary gives the word as hydro- plus plane, with hydro- going back to Ancient Greek hudor, water. English plane carries the idea of a flat surface and of sliding along one, and it is also the ordinary word for an aircraft. Both readings show up in this word's uses.
Wiktionary 記的組成是 hydro- 加 plane；hydro- 一路回溯到古希臘文 ὕδωρ（水）。英文的 plane 既有「平面、沿著表面滑」的意思，也是「飛機」的日常用字。這個字的各種用法，正好分別落在這兩層上面——這一點是本字用法的觀察，不是來源裡寫死的說法。
Wiktionary, "hydroplane"
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