Diesel in the freshwater tank

My fresh water tank is under the dinette seat, taking off the wood cover, no screws, there is a hatch in the top, I drained the tank down as soon as I arriv3ed home after collecting from the dealers, but on opening the hatch, there is still water in there, so needs to be removed as I want to clean out as only a recent buy.
pleased the tank is inside as well as the pipes.

Any ideas apart from soaking it up with cloths, until clear?
best regards,
M
Sorry gone off

You could try a straw Margaret but it may not taste too good, I clean ours out with a sponge and cloth and can't think of any other way!