An oil can is a dent in the hull of boat.

It typically occurs under the seat, it can also happen in the bow and stern from improper storage or from letting the boat sit in the sun on an uneven surface for an extended period of time (i.e. leaving the boat on its hull on a rack).

An oil can does not affect a boat's performance. It can hinder spinning on flat-bottomed planing hull boats, but it needs to be a pretty big depression. It can also slow down a touring boat a touch, but again, it needs to be a big dent.

To prevent oil canning, add foam under the seat to push the hull back into place. To prevent oil canning on the ends try to store the boat on its side. Also avoid tying the boat too tightly to racks, especially in the summer when the temps are hot (plastic is slightly softer in hot temperatures).

To fix an oil can, set the boat out in the sun. Usually it will pop out in a day or two. If it's not coming out, then add foam to push it back into shape.