Hey guys, thanks for all the replies. I was very curious to know this reason and I saw it on the CL-415 as tbuelna pointed above. It was not just at the bottom, but both at the top of the fuselage and the bottom of the fuselage.
I have noticed pretty much all the amphibious aircrafts I have come across use universal head rivets on the fuselage skin oppose to CSK rivets. Would anyone here know the reasoning for it?