Engineering question on steel?

Im building a pontoon boat and i'll have mild steel across the two pontoons that will also hold the floor, i need them to not sag at all in the middle and it has to be strong enough to hold the force of 10 people plus the force from the pontoons being hit by water, trying to spread apart, etc. I know i can figure out what they will support at a certain length if i find the bend moment, by weight supported/2 set equal to the moment, but I cant find the moment of any metal what so ever, any idea on where i can? also, do i need to worry about shear force, i dont know what shear is.