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.
- There is the treasure chest which is the most useful one since it contains necessary iron and gold and sometimes diamonds.
- There is also the map chest which contains a buried treasure map for locating buried treasure for more useful resources.
Shipwrecks also come with some wood logs which may be useful if you need to obtain them.
After looting a shipwreck, depending on the resources obtained you should be ready to build a Nether Portal to head to the Nether.