Before putting a monstera leaf into water, find the piece of stem it came from. A leaf with its stalk is different from a stem cutting that includes a node capable of producing new growth.
Locate the node on the main stem before making the cut. An aerial root may help you recognise the area, but the important point is to retain the necessary stem structure rather than choosing the largest leaf.
Prepare for the next stage
Use a clean container and a propagation method suited to the plant. Keep the cutting supported so delicate new growth is not repeatedly knocked against the vessel.
A leaf remaining green does not prove that the cutting can develop into a complete plant. Record what material you took and watch for roots and new growth without constantly removing it for inspection.
Plan the move to a growing medium before the roots become difficult to handle. A cutting needs appropriate aftercare as it changes conditions. The first useful decision happens before it enters the water: whether you have taken a viable piece of the plant at all.
University of Minnesota Extension states that a Monstera deliciosa cutting must include a node. That is the relevant distinction when deciding whether the material can produce a new plant, rather than merely keep a detached leaf looking fresh.
Related reading: Monstera Cuttings: Nodes, Rooting and Potting On.