It isn't enough for you to simply prove that the mechanic started your motor in the ordinary course of working on the boat. In order to prove them liable, you would have to prove that 1) the mechanic or marina negligently
caused the water to get into your diesel
fuel, or that 2) they started the motor, knowing or having reasonable cause to believe that there was water in the
fuel, and that it might be damaged by starting it.
They're not liable merely because something bad happened to your boat when they started it. In order to prove them liable, you have to show that they either caused the damage, or that they started the engine, knowing, or having reasonable cause to believe, that a condition existed that would cause damage to the motor if they started it.
Good luck!