Hi, whitenovember,
From your post, I understand that after going to the beach, you noticed a white line down the side of your iPhone display. I'm happy to help!
While at the beach, it's possible the iPhone came into contact with water or sand, or excessive heat, that may have caused the issue with the display. These scenarios could have a similar effect on any electronic device that has a display, and I would encourage you to set a reservation and take it into an Apple Store or Apple Authorized Service Provider to have it looked at. A Genius or Apple-certified technician can definitively determine the exact issue and steps to resolve: Apple - Find Locations
Your manufacturer warranty covers defects that are a result of manufacturing. If water, sand, heat, or other outside factors have caused the line down the display that would not be considered a manufacturing defect (as it was not caused during manufacturing). In that instance, the warranty would not cover the issue, however you may be able to get a repair done at a cost less than the purchase of a new device: iPhone Service Pricing - Apple Support.
Cheers.