The United States forces brought with them "magic pontoon boxes" that could be bolted together to form long piers and docks. These would extend all the way from the shoreline out past the reef where transports were anchored next to them and could unload what was needed, including vehicles.
Here is an excerpt from "The Battle of Saipan -- The Final Curtain" by Cdr. David Moore, USN (Ret.):
"With this invasion fleet came the prowess of the American industrial giant. Perhaps an excellent example of a war secret was the thousands of Admiral Morell's magic pontoon boxes. Neither the Japanese at Saipan nor the Germans at Normandy could quite match this American ingenuity of producing ideas and war materials. And so it was - the Seabees devised a steel cell (5 x 5 x 7 feet) which could be bolted at the corners to form unsinkable barges, piers, or other invasion units by simply rearranging the magic cells. These had 1.5 feet draft sufficient to clear the reef lying 3 feet below the water, which extended around Saipan. The Japanese knew their boats could not clear this reef, and with coastal guns in place, they felt safe."
I would suggest that you go to Google Search, enter "LSTs at Saipan" in the search box and you're going to get quite a few hits.