Shipwrecks are generally the fastest method of gaining resources to prepare for the nether since they consist of at most 3 chests that can be quickly looted.
Generally you will want to go for shipwrecks when you spawn on an island or near an ocean and useful items to craft when going for them are a boat for travelling through the ocean fast. Also be on the lookout for dolphins which allow you to travel faster than boats. There are 16 different variants of shipwrecks and they spawn with different chests so being able to identify them quickly is a useful skill.
In a full shipwreck, there are 3 chests:
- The supply chest or food chest which is useful since it can have food like carrots, potatoes, suspicious stew, wheat for crafting into bread and rotten flesh if needed. It is found below the deck, near the front of the ship.
- There is the treasure chest which is the most useful one since it contains necessary iron and gold and sometimes even diamonds. This one is found near the back of the ship, under the deck.
- There is also the map chest which contains a buried treasure map for locating buried treasure for more useful resources. This chest can sometimes appear at the back of the ship, above deck, in a small room.
Note that chest locations are relative to the bottom of the ship, not to where gravity falls. Many ships are rotated on their side, or even upside down
Shipwrecks also come with some logs which may be useful if you need more wood. Note that mining them can be difficult and dangerous in water and if you want to play it safe, build a door, then enter the air space it formed, and mine the blocks you need (only Java).
After looting a shipwreck, depending on the resources obtained you should be ready to build a Nether Portal to head to the Nether.