I dropped my iPhone 7 Plus in the lake and now the screen has lines

As Johnathan Burger has already mentioned, your iPhone had the lake water enter into the device. Although it may eventually dry, it will leave conductive debris inside the phone. Any further use of the phone may introduce further damage. You also will increase the likelihood that the lithium battery inside the phone can catch fire or explode if you continue to use it.

You basically only have two choices:

1. Have an Apple Technician evaluate the phone to see if there is any possible means to "repair" it. Most likely not a real option, OR
2. Replace the phone.

Buy a new phone.

Water damage is not covered by any warranty.