Bremerhaven's main sights are unusually concentrated. The Havenwelten waterfront holds the emigration museum, Klimahaus, zoo, maritime museum, submarine, harbour views, and enough cafes and fish stalls to make a full day easy.
Give the museums time. The German Emigration Center and Klimahaus are both substantial, and trying to rush every attraction into a few hours makes the city feel thinner than it is.