The extra fin compensates for all that float area sticking out ahead of the CG. Without it, stability about the vertical axis is diminished, and it's easy to spin the aircraft without intending to, and without the extra fin, you might not be able to recover. This could be the source of your instability on the water. I would almost bet on it.

Once you get "gradiated" from this seabird, I highly recommend going to some kind of low wing float plane. You'll find that almost anything high wing suffers from water penetration into the electronics area, because the floats (and hull on flying boats) send a tremendous jet of water against the wing saddle area. This is just not a problem with low wing float planes. The water bounces off the bottom of the wing. (See pic of my Heinkel 31, built from Ed Westwood's plans via RCM magazine.) My next sea bird, to be finished some time next year, is a 60" wingspan P-51 pattern ship on floats. Sorta like an American version of the last Schneider Cup winner, the S6B Supermarine racer, which was the basis of the Spitfire, the airplane that saved the world. I'm going to color it like the Reno Racer, "Miss America".

You're right - the floats on my Star are big. probably bigger than they need to be, but the water handling is superb, and the only times I ever flipped it over was when I was being stupid and not landing or taking off into the wind. It is kind strange having a trainer A/C with a wing loading of 32 oz per square foot, though.