Your life could depend on staying warm when wet. Fit with two pair of wool socks. Buy boots that fit your needs. Many people over-buy, assuming bigger means better. Big, heavy boots have their place, but for most hikers, they are a burden and a waste of money.
Heavy boots are also harder to break in.
Above all, boots must be waterproof and comfortable. Blisters can be a painful part of your Yosemite experience. It is important to prepare your boots and your feet for hiking. Wear your boots for half-day periods for several weeks before your Yosemite trip. This allows boots and feet to get used to each other.