As suggested elsewhere for you you can organise the propshaft at an angle to raise the inside end above the waterline.  When my son and I built his first boat, I barely knew much more than he did , we organised a couple of meccano pulley wheels to gear down the motor drive to the propshaft and it also permitted the motor to be on the bottom of the boat with the inner end of the propshaft up above it.  The small differences in angle were handled with rubber band connection.

Propellor … I would buy myself a length of 3/16 brass tube, comes in 12 inch length from model shops.  Also a length of 4mm rod.  Get a bit of tinplate from a food can and cut out the shape of a propellor and bend blades to an attack angle after drilling a hole in the centre for the propshaft.  Double bolt prop to shaft.

I trust you have a 4mm tap to cut the thread on the end of the propshaft and can solder a vertical pipe for the oil as I suggested elsewhere.

To make a flexible coupling I suggest a short length of curtain wire with brass couplings gribscrewed to motorshaft and propshaft,  I use 3/8 Brass HEX and 3mm grub screws after drilling appropriate holes to suit the three shafts [prop/curtain wire/motor].  A pair of grub screws on opposite sides of the HEX.

I follow these principle irrespective of wether I am short of the ready or not … I like doing it myself

The problem with this practice is that innitially you can spend more on tools than to buy the item .. but tools usually last and last if they are any good.  But I am reminded of the Model Engineering Guru "LBSC" who as a child built working live steam models with a file and a knife he had filed teeth into for a saw. along with a soldering iron.  He had the creative ability but no money.