The use of a longshaft/shortshaft has to do with the height of the boat transom (where you 'hang' the outboard motor). 15" or so is a shortshaft, 20" or so would require a longshaft motor. When you run out of water, it won't make much difference if you have a shortshaft or a longshaft as they will both be about the same depth in the water below the boat bottom IF you place the proper length shaft on the proper height transom.
As to whether a 10 hp. outboard would correctly power a 10 ft. boat, a lot of this decision is based on what the boat manufacture recommends. You could keep an eye out for what other folks use for power on their 3m inflatable...
Hope this helps.
