As for water. . . .it's your whisky, drink it as you please.
That said, the simple chemical truth is that adding a bit of water does slightly lower the alcohol level which means that certain compounds will find it more difficult to stay in solution. The flavor and aroma of the dram will change for certain as those compounds come out of solution to some degree -- most consider it to improve the flavor and aroma, but you decide.
You can easily see this with a simple experiment of your own, two drams side-by-side in identical glasses, one with a tablespoon or less of water in it, the other neat. You will be able to discern a difference. You can decide which you prefer.
That said, the simple chemical truth is that adding a bit of water does slightly lower the alcohol level which means that certain compounds will find it more difficult to stay in solution. The flavor and aroma of the dram will change for certain as those compounds come out of solution to some degree -- most consider it to improve the flavor and aroma, but you decide.
You can easily see this with a simple experiment of your own, two drams side-by-side in identical glasses, one with a tablespoon or less of water in it, the other neat. You will be able to discern a difference. You can decide which you prefer.