Hmm... The only island cave I can think is the one to Regice, right? No? Maybe... I don't know what you're asking! Getting off subject, anyways...
WARNING: The next paragraph contains content only to open the three caves to the regis (you know, with the underwater ruins by Pacifilog town). If you've already done so, skip next paragraph.
To open up all three caves you need:
A Relicanth and Wailord
A Pokemon that knows dig, surf and dive
Start off in Pacifilog Town. Surf towards Slateport City. You'll notice there's a lof of fast currents there (hard not to), so stay on the bottom part of the currents. You'll eventually come across a calmer area with deep water. dive down, swim around, and you'll eventually find some braille writing. Read, and resuface. You'll be in an entirely new area. You'll be in a room with the locations of the regis (all in braille, of course). Walk to the north side of the cave (just go up) and head to the middle braille. Use dig, to make a hole in the wall, and go through. Head to the middle slate in the next room again, and then put the Relicanth in the front of your party, and the Wailord in the back. Press A, and there will be an earthquake, opening the doors to the regis.
If you've done so, good for you. So, the island cave, right? Regice, I hope, anyways. To open door inside, very simple, no? Just read braille, then run in a clockwise circle around the edge of the cave.
4 island in icefall cave to reach lorelei you need to find HM7 inside icefall cave.
Go to four island and go into icefall cave in there find HM7 use it to go up the waterfall in the cave go to the ladder you will find team rocket and lorelei beat the rocket grunt they will leave then you should leave to six island head to dotted hole and the guy won't be there use Cut on the door to get inside.
By cave floor do you mean maniac cavern? If so then catch all 26 unown
you find raquyza at the sky pilliar
Go to four island and go in icefall cave find HM7 then use it to go up the waterfall go to the ladder then defeat the rocket talk to lorelei then leave and go to six island go back to the cave and the guy will not be there use Cut to open the door.
The sapphire is on Six Island. You need a Pokemon to use cut on the dotted door of the cave.
Go to lilycove city and surf right then up into a cave by the sea. Then go through the door that is above the room.
It is a little north west of Lilly cove you will see a cave on water with a door everywhere
You read the middle braille, head for the middle of the room, and use fly. the door should open, and you'll be able to fight a level 40 Registeel.
4 island in icefall cave to reach lorelei you need to find HM7 inside icefall cave.
save lorelei on four island she is in icefall cave you need to get waterfall to save her which is inside the cave.
Just In the second door upstairs the Pokemon center.Answerwhen you give the ruby and sapphire to Celio AnswerYou need to give the blue and red stones to Celio on island one in Pokemon FireRed and in Emerald you need to have beaten the champion and elite four.
USE SURF AND GO TO THE ISLAND IN THE MIDDLE, AND ENTER THROUGH THE DOOR.
Read the inscription and then go to the middle of the room and use Flash. The door will open and you can battle Registeel at the usual Lv. 40
first you fly to dewford and surf north (route 105) there will be a hiker on a island on the left side and surf past him and then there will be a swimmer surf up past him and there will be a little island surf on that walk up to the middle stand there for about 3 minutes and the door will open (you must have wailord and relicanth
Head to one island and go to mt ember and get the ruby then deliver it to celio then go to four island and go in icefall cave find Waterfall HM then go up the waterfall save lorelei then go to six island Cut down the door on dotted hole and go in and find the sapphire then deliver to celio now you can trade with ruby or sapphire or emerald.
You can find the sapphire in the dotted hole cave on six island to find the cave go to the southern portion of the island, use Cut on the door to open it then fall down the hole in the cave then fall down the holes in this sequence: up, left, right, down.