Not sure what you mean with side supports, you mean to take the floor?

 Can you post some pics? I know in some dinghys the bench seats are there for structural reasons. 

 Yeh the benchs are the support for the sides thats why im asking for some ideas of supporting the sides without the benchs =)

 I think then you'll require ribs up the side welded to the gunnel rail. Check out any bare dinghy's and you'll see what I mean.

 if you intend to make the height of the new false floor the height of your existing chines--the extrusion where the sides meet the bottom--just weld a framework of 20mm square tube across the chines  at appropriate spacing. Then weld longitudinal pieces in, placed to suit an access hatches you may be thinking of adding. Then extend the transverse frame pieces up to the gunwhales and weld, only at the top where it hits the gunwhale and the bottom where it is attached to the transverse frame. Welding to the actual skin could result in tears--these things are meant to flex a bit.  Bit harder if you want the floor to be lower than the chine , but principle is the same. it will be stronger than original. 16mm marine ply is plenty thick enough for flooring, as long as you don't try to skimp on materials and welding by spacing your crosswise supports out too far. just from looking at your picture ( sideways), I would have each of the transverse frames  above the existing frames on the bottom, and weld a vertical support beween the keel and the frame. I've been through this, partially, modding a big tinny that still had the thwart seat up the front, and it worked out really well.