In the Emerald game section of the site it tells you about Shoal cave, so does the Prima strategy guide. Like saphire said, Shoal sand can be found when there is no water in the cave. Bring a Pokémon that knows Strength so you can get past the boulder that lets you access the bottom floor which is 50% ice. On that floor, on the walkable sections, you can find Snorunts from time to frequent time. Note that it is not a water type, only an ice type, so use Ultra Balls, not Net Balls.
As for the Abandoned Ship, on the 2nd floor, through the middle door on the bottom, there is a hole in the floor full of water. Dive in it (P.S. Using the Super Rod only here and the other end will very rarely bring you a Tentacruel, although Tentacool is found everywhere and evolves at Lvl. 30) and go to the other side. Go around until you find an unlocked door and enter. Watch for sparkles on the floor. 75% of those are items, mainly keys to the doors. The game will say whether or not it is an item, and which room key it is. The last room contains the scanner. Return to the scientist and he tells you to go see Capt. Stern in the port in Slateport City. (P.S. The key to the storage room is next to the scientist. In the storage room is the Luxury Ball. Be careful it's the only one... mine caught Regirock)Go see Capt. Stern and he gives you the choice between DeepSeaTooth and DeepSeaScale as a reward. Tooth increases Clamperl's Sp. Atk and Scale increases its Sp. Def. Trading with Tooth gives Huntail, while trading with Scale gives Gorebyss. (P.S. Go back to the ship afterwards and the scientist tells you that the ship was the S.S. Cactus. But that's irrelevent in the game.)
That's all.
