Clothing should not be too tight or too loose. Clothing below the waistline should be darker than above. Otherwise, even light jeans worn below a darker top could easily attract attention to the lower part of the body and away from the face. For a beach portrait session it is sometimes a good idea to dress in light colored clothing, so that the bodies will blend with the light-colored sand. In this situation all white or pastel clothing works fine.
All white clothing is great.
White shirts and jeans work well, too. White shirts with beige pants is even better. Staying all in light tones can be very effective in this type of environment. Still, remember to dress everyone in the portrait similarly. You have to blend the bodies of everyone together, so that the faces stand out.