Montana- you may just plain ol be too durn heavy- once you get the floats set right - step under cg and a slight down angle to th plane's nose setting on the water -- try this:
on a calm day try a full power tke oof with only enough elevator to bring the plane up to apparant level position - if it is not too heavy it will , at that point , gradually take off .Adding more up elevator will just dump it back into the water.
larger props don't make more power -they make less -
leave it a bit rich and try a APC 11x6 these won't split when the water hits em
unless that engine is cranking 12000 -it ain't at full power.
Are you flying from a mountain lake (high altitude)?
I was born in Moiese- and that is a lower alitude -for some of Montana-
more altitude and you get less power - so you can't swing larger props and the plane must be lighter -
flatlanders really have to se how much this affects things to really appreciate it.