I stretched the duck width-wise to make it wider and more stable in the water. I think it also looks better this way since most rubber ducks I've seen are quite wide and look almost round from above. The attached 3mf file is prepped with infill settings for best floatation. The modifier slab is set up to make a solid infill layer that gets thicker toward the back to balance the weight of the head. The rest of the infill is minimal to reduce weight.

Update: Added STL without modifier block for those who just want a duck.