@srujanakoneni yep, that is what happens when your expensive AMOLED phone gets water damaged. Only you will know what you are going to do with this. I am not sure where the "around $300" price comes from or what it entails. With that out of the way, and you are on iFixit looking for help, that leads me to believe that you are thinking about fixing it. Good choice. After all, the idea behind iFixit is to empower you to repair your own device.
Let's start with the basics. First thing that you want to do is to stop continuing to try to charge, sync or otherwise use your phone. This potentially can make a bad situation worse.This may further damage your phone.
The next thing to do is to clean it. Disassemble your phone using Samsung Galaxy S21 Ultra Repair for that . Next you must remove all EMI shields from your charging board as well as the motherboard. If you do not remove those, you might as well not do anything. Once you do remove those, clean all the boards, connectors on the board as well as on the ribbon cables etc. with sterile water to get rid off the salt and debris from the water it got submersed in. Do not use regular drinking water.
Once you have cleaned the gross contamination, use +90% isopropyl alcohol . Follow this guide. Even so it was written for an apple iPhone 3G all the points are still pertinent to your phone as well . While you clean your boards, check for any obvious damage, like burned or missing components etc. You do not need to leave it to dry for any time. Isopropyl alcohol in the higher concentration, will evaporate quickly and dissipate the water. The importance here is the cleaning.
Once it is properly cleaned, replace the battery ( and most commonly on your phone the charging board as well). All these steps are to avoid delayed failure caused by corrosion. Best thing to do would be to clean it with an ultrasonic cleaner, but if you do not have access to one, this will at least help. Once all this is done, reassemble your phone and reevaluate. Until it is cleaned, everything will only be a guess. It is possible that you have a bad display and replacing the display assembly may fix it. Replacement screens are available at multiple stores online. You can also get less expensive LED screens. Not quite as nice as the AMOLED but they are usually about 1/2 price. Just make sure that you get a display for your version of this phone.
SM-G998U1 (USA unlocked)
Even with all this, water damage has only a small chance to be fully repaired, but at least you can give it a try.
Now, if you just want to replace your phone, your Google search is as good as ours, and you do not need advice from us on that.
Repair is War or Entropy!