I've crossed the channel a fair few times in powerboats, and I have to say hitting stuff worries me more than anything else. Nearly every time I've crossed we have either almost or actually hit something. If you can, always have somebody with eyes on the water in front of you!
In the channel the separation zone between the shipping lanes seems to be where you need to be most alert - in there we have hit pallets, railway sleepers and bits of floating netts before. (and have had close shaves with oil drums and smaller blocks of wood).
The worst part is, as you hit the object the massive bang! and then watching the engine revs fall as one or both of the engines almost stalls, then not knowing what the hell has happened and thinking: oh ****
we're going to sink in the middle of the channel!!
The sleepers and pallets don't tend to do much good at 22knots (you know what they are when you see the two halves bob up in the wake of the boat out the back) - on both occasions they have bent blades on the props causing huge vibration through the whole boat, and reducing our speed to about 10knots for the rest of the journey. We were fortunate on one occasion to be lifted out at cherbourg (on a sunday) where a highly skilled chubby french bloke with a big mallet 'tapped' them back into shape - they then ran smooth enough to get 18 - 20 knots on the way home.
I've heard the big buoyant containers that ships 'loose' in bad weather often float just under the surface - you really would be pretty unlucky to hit one of those at any speed, but also hopefully very unlikely!
Around local coast, lobster pots are the biggest pain in the arse, but they normally plant them in lines, so once you have spotted the first, you can normally avoid the rest. I've never heard anyone suffering real damage from these, apart from having a ball of rope around the prop - which if you have outboards, shouldn't be too hard to sort.