Put oven racks in upper and lower thirds of oven and preheat oven to 450°F. (If you are making just this dish, you can put oven rack in middle position and roast vegetables on that rack throughout.)
Toss carrots and fennel with olive oil, water, fennel seeds, salt, and pepper and arrange in 1 layer in a 17- by 11-inch shallow baking pan. Cover pan with foil and roast vegetables in lower third of oven 10 minutes, then uncover and roast, turning occasionally, 10 minutes more. Switch pan to upper third of oven and roast until vegetables are tender and browned, about 10 minutes more.