Robyn,
I am far from an expert on float planes, I have 3 right now and only my Cub has a ventral fin. I have a Pacific Aeromodels Knockabout and a ModelFly Cessna both on floats without fins and they both fly fine. I would guess if the SE flies good and seems to have good rudder authority in turns, it probably doesn't need the fin. Maybe some others with more experience with and without fins might like to pipe in here.

Russ