Froggy, please see the attached snips from a 1937 Modern Mechanix plans book. If anyone wants the whole of these plans as a PDF, then pm me - but remember to give your email address as the 2.7Mb PDF is too large to send via this forum.
And please notice in the second illustration how the lady is waving wildly on the deck - this is because her husband who is supposed to be steering the outboard round the back has sat down in the lawn chair to read the sports pages.....
Backing the trailer into the water and then adding the floats/pontoons doesn't work well - the water needs to be more than 15" deep for the pontoons to float so by then water is coming in the trailer door and all the chassis has been soaked.
One rather impractical idea that I had to get round this is:
- reverse trailer down slope into water until just the tyres (and not the brake electrics...) are in the water;
- put on thigh waders and walk into water;
- set up axle
stands in the water;
- put long planks on seabed behind wheels with ends on the axle
- reverse trailer wheels along planks to end;
- add pontoons as trailer is now in deep water.
Not reversing off the side or the end of the planks would be a stylish addition to this procedure.