Congratulations on your purchase. Now speaking from experience my diesel lift pump developed a leak and put about 10 litres of diesel into the bilge , I also found a weep from the diesel tank when I finally removed it. The boat reeked of diesel it got into every thing. The first thing is find your leak it's a waste of time doing anything till that's done . Now the hard part . I got some jerry cans from Bunnings and cleaned out the bilge into those for disposal ( you can't just dump it into the ocean) . (I now run the orange bilge cleaner through every 6 months )

Next i I washed everything down with plain dishwashing liquid and a brush ,rinsing with fresh water from a garden sprayer. Repeat,repeat,repeat .lemon smell the nicest in my opinion. Your boat will probably still smell of diesel cause it's now into any thing that will absorb smells . Remove everything off the boat you can ,wash down all the ceilings,walls,floor I used sugar soap for that .Open every hatch ,cupboard ,floor access and let the air flow through.

Time and ventilation is now your friend , I set up a 12volt computer fan with a duct from the engine compartment ( that's where most of my smell came from ) taped onto a vent that went outside and left it running 24hrs a day .Also had the solar powered vents set to draw in air at the bow and expel at the stern. I made up a spacer for the front hatch that kept it open 50mm and was still able to lock it.

I dont know know if it helped but I also sprayed Glenn20 disinfectant around because it also keeps mould away if your boat doesn't have much ventilation.

I threw out my cushions as I was replacing them any way. Under the floor area was a problem as the boards were not sealed and they absorbed the diesel smell , I kept all the bilge access open and moved my fan around ,every week it got better. You will know when it no longer smells when you can come home from a weekend and smell your clothes the next day.

I wont tell tell you how long it took ,but as winter is coming up and you probably won't use your boat every weekend you should be fine for next season. If any one else has a better idea or answer I wish they had told me two years ago lol.

i am replacing my engine soon and I'm so paranoid about spilling diesel now I have the oil absorbers ready to line the bilge before I start.

Bunnings is your friend.
Glitz Citrus spray
Buy the big container.
a lot better than Domestos

Beeing an old truckie, l prefer diesel smell to petrol smell by ten to one.
That does not mean l use it as an aftershave so getting rid of the smell is a chore.
Soapy bilge wash few times then acid wash with well dilluted hull wash (phosphoric acid or oxalic acid) then soapy wash again (be surprised by the shine on all S/S bits you washed like keel bolts and verdegris on the copper bits) then use a liberal amount of teetree oil or similar again and again and...
Run your bilge pumps a lot because that is a trap for any diesel!
The use of fuel absorbing cloth is a good idea, l use it all the time but a full immersion in diesel is a bit too much for any amount of cloth one might have.

The aeration of a yacht is easily achieved by leaving the air circulating all the time and leaving the bilges open. Opening all doors if any and the bilges too plus leaving the hatches open a bit and cover them with a meter by meter canvas tied to the gunwales with bungee cord to keep rain out is my way of keeping circulation going all the time. Even in the biggest storms it is working, proven in the last few years.

I have had numerous boats with with oily bilges, either the sump is not easily accessible so ive drained oil and then tried to remove the oil later, BIG MISTAKE. or there is diesel leaking from fuel system..
As others have stated, the oils/fuels in your bilge mixed with some water will grow a bacteria that is going to "outlive religion".
To avoid this i know use a vacuum oil transfer pump (have become quite cheap nowadays under a 100 bucks on ebay) to remove all fluids, and place rags on any suspect leak points until leak has been rectified. Those oil absorbent pads are also really handy if u can find/afford them.

To Remove the mess and smell ill pour a diluted mix of "ct18 bilge cleaner" from supercheap.. detergent also works ok..
and leave it while i go to take away shops or restaurants and collect their old oil drums.. then use a small bilge pump that sucks from lowest bilge point to fill drums....take to local supercheap or oil collection point to dump for free..
Onboard I have a small pressure washer that only flows 5.5lp/m at 2000psi (also under 100 dollars) after spraying area with a mist of bilge cleaner i blast engine and bilge down and can usually remove all the oily water in one more 20l drum. Hot water works wonders here (black hose on deck)

One final wipe over with a rag and some more bilge cleaner and the bilge smells great..

cushions and everything else that may have absorbed the smell.... well, good luck.

Ventilation, washing , scrubbing, febreeze, onions cut up in bowl of water..

a job i hate so normally get the first year apprentice to do it . use a detergent based degreaser mix around bailout till bone dry then redo if you don't get the lot out the remainder will stay in the bilge hose and keep reappearing