Peculiar problem! Only turns ON when heated.

With water damage, you have to decontaminate the logic board before doing anything else, otherwise there will be, as you have seen, latent issues down the road.

The water is inside the phone, on the logic board and under the shields, even under the IC's. The real problem is the mineral deposits that can cause short circuits or the corrosion that is taking place as the water evaporates. Leaving the power on the device accelerates the process. The longer you wait, the more time you are giving corrosion to damage your logic board. The saltier or harder the water is, the more damage will occur. The water needs to be displaced, not evaporated. Here’s a great video that summarizes these issues.

• Open your phone and remove the logic board (follow this guide)
• Inspect the logic board, especially around the connectors and look for corrosion.
• Inspect both sides of the board. Unfortunately, most of the board is covered in shields. That's usually where the damage is occurring.
• Put your board in a container with >90% isopropyl alcohol and let it sit for a while.
• Use a soft brush, like a toothbrush and lightly brush away any corrosion you see.
• Rinse in alcohol and repeat.
• Let it air dry for a day.
• Re-assemble and hope for the best.

A professional repair shop that does water damage repair may be able to recover your phone or the data because they have access to pro-level ultrasonic baths and specialized cleaners as well as the skills to troubleshoot your board. Many shops have a no fix/no fee policy so you don't have to spend money to find out if the phone is fixable or not.