Question

What causes much of the east-southeastern part of the country to experience warm, humid, and marine west coast climates?

Answer 1

The Gulf Stream, a warm-water current in the North Atlantic Ocean, provides both warmth and humidity.
Warm water evaporates faster and warm air holds more moisture than cold water and air.

Explanation:

The Gulf Stream flows from the warm, semitropuval Gulf of Mexico along the southeastern coast of the United States, then through the North Atlantic Ocean. It even affects the climate of parts of Europe! See here for more details and some thermal images.