Starting a diesel engine in the course of commissioning a boat for the season is not an uncommon or unexpected thing. Unless you can prove the water in the diesel tank was the fault of the marina, or that the marina had reason to suspect there was water in the fuel tank and started the motor with a "reckless disregard" for whether the water was there or not, you will not have a case against the marina.
If the water in the fuel tank was caused by something other than the marina's actions, like your negligence or poor maintenance, and you did not mention it specifically to the marina staff working on your boat, you could be found negligent yourself. In most states, if you have been negligent and your negligence is found to have contributed to or caused the damage, it is very difficult for you to win a negligence suit against the marina.
You do not mention how the water managed to enter the fuel tank. You do not mention whether you had spoken of your concerns about water being in the fuel tank, or even if you had any such concerns.