Just plant your cutting in a container and put a clear plastic bag over it.
The soil needs to be moist but never soggy and extremely well drained: some people swear by a 50/50 mixture of peat and sand (or akadama) but personally I prefer citrus potting compound. Keeping the bag on will help retain moisture in the ground for a long time.
I've never seen Japanese maple cuttings in plain water honestly so I cannot vouch for it.
