Boomer
From KayakWiki
A boomer is a wave that breaks on a shoal. If a single wave in a train is somewhat larger than the others, the smaller ones will pass over the shoal without breaking. When the big one hits ths shoal, it can break suddenly and the sudden break will cause a loud boom - hence the name. A boomer is a risk to a paddler, since paddling over a shoal might suddenly expose the paddler to a violent breaking wave.
