Phone won't turn on and Motherboard fried can I recover pictures?

Hi Clare, not sure if it is too late regarding your phone.

The only way to recover the photos is to get the phone nominally working . It doesn't have to be fully functional but enough so that you can sync it with iTunes.

This will require a proper decontamination to displace the water and remove any corrosion that could be shorting components. Once that is done, it will need to be troubleshooted to find what, exactly , was damaged by the water and have those components replaced. Some phones are irrecoverable but when precious photos are in play, it is always worth a try. Hopefully next time you will maintain an up to date backup of your device.

I would recommend that you seek out a qualified repair shop that does water damage and micro-soldering repairs. Apple will not help in this case.