I have used Bilgex to good effect. Fishermen swear by it for cleaning their boat’s bilges. Not sure of its environmental credentials, but according to the www, it’s still widely available. I’ve got a rather battered and disreputable wet and dry vacuum cleaner, which is great for sucking the ‘orribe solution from the bilge – but a simple bilge pump would do just as well.
Then there’s always the car engine cleaner products you can get at Halford’s and the like.
And if you value the clealiness of the finger nails, invest in a pair of Marigolds!