A fill flash or reflector, makes the face look natural. Underexposed shots are caused by large areas of white sand, whitewashed buildings, white yacht hulls and so on. The sun drops quickly in the tropics. Try magnifying the sun with a 300mm lens. Expose for the sky near the sun.
Salt water is a killer of camera gear.
Sand is best shot low to the ground with a wide angle. Wait till the sun is oblique to give texture and a 3D effect. Take care to avoid sand penetrating into the delicate internal workings of the camera, lens and shutter. Aerial haze caused by high humidity often found in the tropics can be cut to a slight extent with a polarizer. Best not to expect good results of distant land and seascapes, particularly with prints.