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Typically if he is in a quiet bay you would need to suggest he has paddle out a fair way as sharks dont come into shallow water.
Generally shark attacks occurs by being attracted by movement, so maybe a seal swims past and as he swims back to shore on the board the sound and shape of him swimming back on the board attacks the shark who was seeking out the seal, as the shape from underwater would be similar.
Have a look at the video here, where a shark does kill or attacks a human they go for a limb its often most lose an arm or leg, which just get ripped off by force, no sooner than the shark takes a bite it will release as it can taste it doesnt have the level of fat it was expecting (from a seal), the same is true when they go for surf boards they bite realise nothing good and they will often swim away.
Where people vanish, its often where sharks bite and swim down into the depths with them as by that time they dont have to worry about other predictors eating their kill. Peoples clothing have been found in sharks stomachs in the past. In the case of divers whom have vanished, often just a flipper is left, and their entire gas bottle, wet suit etc is never to be found.
With the person in the picture in mind your probably more likely to find a limb which was torn off being washed back up to stream than pieces of the swim wear.
If the the entire body is not consumed by the shark at which point the swimming trunks would be digested, then it may be the torso is eaten by other large fish and the trunks ripped to shreds, as the material is so fine they wouldn't be much left..