Well there's a curve ball for you...
Was there water in the tank? And do you have access through an inspection port of any kind? What I'm thinking is that the fuel will float on any water, and if there is any way to skim it off before it gets mixed in any way, or comes in contact with more of the tank and plumbing, that would be good.
Otherwise I'd probably try to pump it out from the deck fill, lowing the suction hone in just enough to draw from the top surface as much as possible.
Also shut off any valves where the water gets drawn from the tank to keep the plumbing system from getting contaminated.
Once the tank is empty, I'd fill with water and a soap that breaks down oil. I think Dawn is supposed to be good. Fill full, soak, drain, repeat. As the fluid level drops, the sides of the tank will get coated with any residual diesel, and that will need to get cleaned, so important to fill the tank all the way to overflow when cleaning. If you have any sort of inspection port it will be a lot easier, but I suspect you don't.
Probably also remove the fill hose and wash that out of the boat so you can get it really clean. Also consider whether the fuel on that hose will have damaged the hose in any way, or permeated it. Maybe just replace the hose to be sure.