Does Nucleophilic substitution require water to happen?
Keeping other parameters fixed, an elevated temperature may enhance the rate of elimination such the product of elimination is more dominant than the one by substitution.  But there is not a "critical temperature" above which only elimination will occur every time, rather than the structure of the substrate and of the reagent equally influence this.

For the other part, transesterifications as one example work better in absence of water.  See, the oxygen present in the water molecule, with highly concentrated electron density, it may act as a nucleophile, too -- a rather hard one (HASB princible) -- leading to parasitic side reactions.