Ah, cool. Most of the people on that site are doing sealed underwater pod motors (much like a trolling motor) and even though those projects are all hydrofoils,

Ah, cool. Most of the people on that site are doing sealed underwater pod motors (much like a trolling motor) and even though those projects are all hydrofoils, a lot of the same advice for waterproofing brushless motors applies for a conventional boat. FWIW here's a really cheesy video for an electric outboard project I did back in college: https://www.youtube.com/watch?v=Elw9DDL6wTM https://www.youtube.com/watch?v=Elw9DDL6wTM