Rainbow colored bilge?

When we leave the lake, we pull the plug when the boat is on the trailer. We pull the plug every time and is part of our wipe down routine. I have noticed this season that something is polluting the water in the bilge. The water has a rainbow tint to it. What is it? Is it gas, oil, grease....? I thought that it might be gas from a recent overflow when pumping, but after a couple of weeks and riding four to five times a week, it is sill a rainbow colored bilge leaving the boat. Any ideas? The boat is a '05 Supra Launch 21V.