It's easier out of water
but you can inspect them and the hoses for leaks, double-clamp the hoses if they aren't already clamp with stainless hose clamps, lube and work the handle to keep it from freezing up, place or tie a wooden emergency plug by each seacock, and maybe wire each seacock together for bonding against lightning strikes. I would consult a good boat maintenance book for more details and ideas.
