The last time I cleaned out mine, I pulled in a hose in, flushed liberally, and used a hand brush to scrub with.
I use a wet-dry vac to get the small amount of water that my bilge pump won't get, which leaves me with a completely dry bilge. Since my cooler drains into the bilge, I use the vac frequently.
Search the forums for painting the bilge if you're looking for advise on that.