Have you looked at trolling motors? They're made for the job and use the water as a heat sink. Have the correct prop directly shaft mounted. Might be your easiest/best solution. Typically many on eBay.
I had considered these, but want to use an inboard motor.
I have contacted the designers of the boat who felt that 500 watts would be adequate: they used a 1,000 watt engine which was more than enough. However, I have looked further afield for a motor and located some manufacturers in China who can supply single 24 volt, 1,200 watt motors, shipped and supplied for reasonable prices, so shall probably pursue that route once I am ready.