1970 20SF Flotation

I'm new to all this. I am told my 1970 20SF has no flotation in it. I can see that there is none under the deck. I am somewhat bothers by the fact that it may not stay afloat if swamped. Is there a way to put flotation in under the gunnels that will keep it upright and on the surface? Someone told me under the deck is no good because the flotation gets wet over time and ends up water logged. Anyone done this before?