I suggest anyone reading this does so with a pirate accent in mind. It's far more entertaining.
If you can get a new mainboard then try it. It could be something else though, have you tried a new battery or dock connector in the phone?
Mainboards are hard to come by as they are basically just a phone minus some parts.
How do you know it's water damaged? If the sensors are tripped then that still doesn't say its the mainboard thats fried. Plenty of people trip their sensors but have fully functioning phones.
Mainboards are expensive - I have a few phones with dodgy mainboards but I have written them off since not economically viable to sort. I have had two phones with dodgy logic and dodgy comms boards and merged them into one. Now that was fun!!!!!