During low tide the island becomes a peninsula. What amazed me was the fluorescent micro-organisms in the water. When you splash the water at night, you see what looks like yellow-green sparks. Sometimes you can pick one up. The riding surface varies widely.
Most places have hard packed dirt.
But you can also expect small to medium rocky roads. This is where the springs of the ranch trucks form waves in the road. I didn't encounter much of this but was warned by other mountain bikers that the road from San Felipe south toward Punta Prieta has five days of washboard with large rocks in some areas. This description was accompanied by various obscenities and an offer to see a saddle soar. You probably will want to take the paved highway for part of the way.