[Q] water damage - need replacement motherboard topic

Hi guys

My z3 died while snorkeling in the sea. Now it turns on when power is connected but keeps power cycling showing only the sony and xperia logos. One time it was showing the battery charging icon and vibrating.

I took it to the shop under warranty and they quoted 0 for a replacement motherboard, stating that they tried it in another phone and it was completely dead.
I couldn't find any prices online and I'm wondering what's the best option.

Any advice appreciated