this is about 3/4 of the way thru the movie when casper jumps onto the boat and starts fighting the guy on board your meant to belive that there is only 2 people (casper and the bad guy) on board but just before the person that is casper van Dian's partner yells out

Look out the rocks!

you see a panoramic shot of the boat in which there is a third mystery person steering the boat

and if you look at the boat right after he yells it out you see that the man has disapeared and there are only 2 people on board again