If there's any oil in the bilge, put down bilge pad(s) to soak it up BEFORE starting any cleaning. Detergents emulsify oil and once that happens you'll play hell trying to have a clean bilge. Replace pad(s) as necessary until all oil has been soaked up. Dispose of the pads in the used oil facility in your marina.
Dawn is as good as any bilge cleaner....you can use sponges, brushes and a wet vac if you want to, but it's a LOT easier to use a power washer 'cuz it can get detergent and water into places you can't reach any other way. After you've scrubbed (manually or with a power washer), it's essential that you thoroughly RINSE all the soapy water and any detergent residue out of the bilge. Would you scrub down a kitchen sink, then just walk away without rinsing? So why do you think a bilge is any different?
After you've rinsed and mopped up any standing water, let the bilge dry out for 24 hours with all hatches open to let plenty of fresh air circulate in it. NOW you're ready to paint.
--Peggie