Very occasionally, one encounters a whisky whose virtues are better displayed neat. Outside the tasting room, many people prefer to drink their after-dinner malts straight - with sound medical justification. In these cases your own saliva acts as the dilutant, and they should be sipped in very small amounts. Blenders nose at 20% ABV, but this can drown some whiskies which tend to 'break up' with too much water. It is always best to add water a little at a time until any nose prickle has disappeared and the sample has fully opened up.
The water you use to dilute the strength of your dram should be still and not too high in minerals. True aficionados will use the water used in the production of the individual whisky they are tasting. Scottish water is predominantly soft, so if your local tap water has a suspicious taste, is heavily recycled or chlorinated, your best plan is to use plain bottled water from Scotland. At professional tastings, distilled water is used.
The ideal temperature at which whisky should be drunk varies according to the climate of the country in which you are drinking it. However, for the purposes of tasting malt whisky, it is best appreciated at the equivalent room temperature of an old-fashioned Scottish parlour (however difficult to recreate in these days of central heating, and hermetic glazing). In other words, you should nose at about 15ºC. Chilled whisky does not readily yield up its aromas and the addition of ice will close them down altogether. On the other hand, warming the glass in the hand - as one does with brandy - helps to release the volatiles in the spirit, especially when the sample you are tasting is neat.