On water reactions are a group of organic reactions that take place in water emulsions, and which have an unusually accelerated reaction-rate compared to the same reaction in an organic solvent or corresponding dry media. Source Wikipedia

The notion of the special nature of water as a solvent for organic reactions began with examples of Diels–Alder reactions more than fifty years ago. The first quantitative data were reported by Rideout and Breslow, who showed that Diels–Alder reactions between nonpolar compounds proceeded at much higher rates in water (dilute homogeneous solution) than in organic solvents. Rate accelerations as high as 200-fold were noted in certain cases."

“On Water”: Unique Reactivity of Organic Compounds in Aqueous Suspension Angewandte Chemie, Volume 44, Issue 21 May 20, 2005

There is a growing list of organic chemical reactions which have been shown to have considerable acceleration 'on water'. It's speculated that the unique properties of molecules at the macroscopic phase-boundary between water and insoluble hydrophobic oils might play a role - but these effects are yet to have a formally accepted explanation.

Also see: Water clustersplugin-autotooltip__plain plugin-autotooltip_bigWater clusters

Water has a long list of 'anomalous' physical (and chemical) properties (list here, archived from London Southbank University). Most are now fairly well understood - but there is currently no overall model to explain all of its unusual properties