Dawn dish detergent, won't hurt a thing and will flush all the oil out. Just mix it up real strong with some hot water slosh it around with a brush and rinse it out. Sometimes takes a couple douche's but it works.

WOW...15 different answers...imagine that

16....

Simple green, rinse with deionized water and blow off with leaf blower. No harm or water spotting.

Dawn dish soap & hot water. Mix it in a garden sprayer. Use the sprayer to get it into all the nooks and crannies. Let it sit then rinse well, repeat if needed.

Zep Industrial Purple is the best I have ever tried - from Home Depot. You can mix it 50/50 with hot water, but be sure to use chemical resistant gloves, otherwise it will take your finger nails off. Spray a small section, scrub it and hose it off immeadiately. The bilge will look new by the time you are done.