iPhone damaged with water, motherboard's problem?

Davide, it may help if you tell us which deoxidant you used. I suggest that you no longer try to turn it on or try to charge it. Disassemble it using this guide and properly clean it. Use this guide to clean it. It was written for a 3G but all the points are still valid for your phone as well. Always check your board for any missing, burned or otherwise damaged looking components. After that replace the battery, reassemble your phone and re-evaluate for any problems. Since you seem to know that you have a damaged dock connector, I would also go ahead and replace that. Hope this helps, good luck.