My Iphone 7 plus unable to power on due after using in water

No, Apple says that iPhone 7+ is water resistant (not waterproof).

But we can't diagnose that issue remotely. You need to have that device inspected at an Apple Store or Authorized Service Provider. After checking it out, they'll give you all the available options: Apple – Location Selector

Regards

Giulio

Your device has been destroyed by the water.

Apple standard warranty does not cover damage from liquids.

Apple will not even attempt any repairs and the device must be replaced.