Scouting japanese held islands pre-invasion

So while air reconnaissance could help give the general lay of the land and help spot bunkers and strong points that could be softened up during a bombardment, what other forms of scouting actually took place. Navy seals came to be during WWII I believe, but I thought they were more for removal of underwater ordinance, whereas today they would go ashore and get any information needed for an invasion. I believe it was the invasion of Tarawa where due to low tide alot of the amphibious vehicles could not make it over the coral reefs that surrounded the island. In some cases I believe we also did not have any current maps of the islands and reefs that were to be invaded as they were just out of the way islands before the war. So, what kind of information could be obtained before an invasion to ensure the bombardments could be succesful, that the troops could even get to the beaches, and that once ashore they would get all the support they needed?

 

Initially the NCDU(Naval Combat Demolition Unit) teams & UDTs(Underwater Demolition Team) were used for clearing the proposed landing areas of any underwater obstacles. However, with the conclusion of Operation Flintlock(the invasion of the Marshalls), their mission was expanded to include beachfront and underwater reconnaissance.

A breif history of several of the UDTs can be found here: http://www.specwarnet.net/americas/UDT.htm

The Marines also sent recon units inshore prior to some of the invasions in the Pacific.

Also, United States submarines would approach Japanese held islands and take many photographs of the intended invasion beaches through their periscopes. These photo sometimes proved to be more valuable than the aerial recon photos in identifying target placed along the invasion beaches.

 

Man, I need to get that UDT page taken down =/
(I used to own Specwarnet but let the domain expire, someone scooped up the domain, scraped the pages from the wayback machine, and reposted it with ads).
I'll get the page re-posted and expanded on my site first though.