If you cant get the phone turned on the only way is to get a very expensive tool that will actually read the data from a NAND flash chip. This requires precise desoldering techniques and the equipment alone is $1500+
You can send your phone in for water damage repair service to a company like RockIT Repairs and they can run it through a heated chemical process to try and properly clean your phone. Also if some pins have corroded they could resolder them to get the phone turning on again.
Let me know if I can be of further assistance.