If its mixed with water a couple of bottles of bilge cleaner from the swindlery is usually enough to emulsify the oil sufficientlyso that it can be pumped overboard. You can also get some absorbent material to mop up the oil as well
'An aquaintance I knew' did thisut a significant quantity of washing up liquid in bilge, stir it up lots, pump into containers, (5 litre ones ideal) Then let it settle for couple of days. Then pump out water from the bottom, leaving the separated oil. Put the oil all together from the separate containers, and dispose of at your local amenity tip along with the other 'old engine oil'. that is surely what it is. Flush away the 'very slightly contaminated bilge water', fairly soapy, but not in the marina or river. A regular foul drain should be ok?
An alternative might be to pump out water from below the oil, then use loads of old rags, newspapers and towels to mop out the bilge. Finally have a go with bilge cleaner. Lousy job to do but good idea to fix the oil leak.
Sometimes even laying some of the absorbing type of kitchen roll on top of the oil soaks it up, then put it in a polly bag that has no holes in it for disposal
Add fairy liquid, stir and then soak it up with cheapo nappies from Tesco..about £1.50 for ten.
I keep nappies in the engine room all the time to soak up odd bits of spillage and change them about once a month.
Remarkable how absorbent they are.
I sort-of think it's a bit late for the environmentally friendly bit, innit? Go offshore and pump the main stuff out , use a small pump and get the dregs out, use paper towel to mop it, use jif or hlb's boat remover or even acetone to clean the bilge to perfection. .
But don't pretend that any of these are environmentally friendly, cos you want to get the oily water out of a large and extremely non-biodegradeable GRP boat, painted with poisonous antifoul, fitted throughout with components that took thousands of kilowatts of energy to manufacture with engines capable of buring fuel at a rate somewhat in excess of the amount needed to pump water or provide electricity for a reasonable sized town in Africa.
Filling the entire boat with topsoil and planting mung beans is probably the best option if you are suddenly very Green. I wd use the phrase "environmentally friendly" very sparingly!