Not sure what to power it with, do you currently have any batteries or anything or are you starting from scratch? A 28x60mm leopard might be a good choice. Motor KV would depend on batteries, should spin a 35-37mm prop no problem. Kind of a half guess though, no experience with smaller electric boats.
Next summer if you're interested there is a group of us who run on Sundays in Peterborough just north of Trent U on River road pretty much all electric, think I'm the only one who still burns nitro around my area, although I'm just about all electric now.