Which is the most common type of debris that litters the oceans?

Plastic.

Plastic will usually float. There are vast areas in the "doldrums" of the oceans where millions of tons of plastic bottles, fishing nets and other plastic garbage are floating. These areas of ocean are called 'gyres' where the currents move around in one gigantic but slow-moving whirlpool.

The largest is called the Great Pacific Garbage Patch, and is found in the North Pacific Gyre.

The plastic gradually breaks down into tiny pellets, which are swallowed by whales and other marine life as they resemble plankton. See the link below.