I would use a 7.2v battery and a 15 amp speed controller 🚤

Hello. I have a similar 400 size motor in my seaplane tender which is also a small boat. see my blog.

I use a Mtroniks 20 amp speed controler and it works fine and doesnt get above 35 degrees. it should be fine with any proppelor size up to 35mm.

for the battery I recomend this one

http://www.modelsport.co.uk/?CallFunction=ShowSpecification&ItemID=29046