Why maleic acid, cyclopentadiene, and water are being used in relation to green chemistry in diels-alders?

Cyclopentadiene is very useful in diels-alder reactions. It is a molecule that is locked in a s-cis conformation which allows for very high reactivity. When reacted with maleic acid (and many other compounds) it will allow the formation of bridged bicyclic compounds.

However, diels-alder reactions do not need reagents such as water, so this does not play a role in what you're talking about.

As in reference to green chemistry, this is an organic sythesis, anything organic is dubbed green.