Why don't we design submersible container ships? Are the energy costs of displacement while moving entirely underwater too expensive?
The wave drag (or wave resistance, to disambiguate from the shock wave drag of supersonic aerodynamics) referenced in sibling comment is the energy contained in the waves created by a surface vessel. The details that you'd have to understand to optimize a hill shape are quite complicated, but for a simple surface vs sub comparison it's enough to know that surface waves exist and that they contain energy that is projected away. A sufficiently submerged vessel does not create those.