Raft

As we were sailing midway across the atlantic, we saw a raft similar to this (minus all the seaweed):

or this:

Unfortunately we caught a fish at the exact same time and we were too busy trying to land the fish to stop and investigate. Fifteen minutes later we had the fish on board (a mahi-mahi) and I decided we would not be likely to find the raft if we turned around as we were sailing over 8knots at the time and in pretty sizeable waves.
I've been kicking myself ever since that I didn't turn around as I'm very curious what it was; it also would have been a great man overboard drill in a somewhat real situation.
So now that I'm home with a bit of time on my hands I decided to try to google an answer. After a serious bit of "googling" I believe I've found the answer. It's a fish attracting device.

Fish Attracting Devices (FADs)

The additional (non)coincidence that we caught a fish at the same time we passed the raft was no fluke (pun intended). I had never heard of these devices before nor had I heard of the threat they are for ocean fish stocks.
You learn something new every day!