Brushless adds a whole new way of thinking about it all. And I could hardly help with a plane let alone a boat. Mainly because the prop in use plays into the final decision. My suggestion is to contact a hobby shop or brushless distributor that specializes in boats and ask them. Personally I'd probably go to your side of the pond and ask someone like Cornwall Models. Just be sure that if you ask someone, you also buy from them. Just good business.
Just knowing what boat it is, a good distributor can probably recommend a complete brushless drive train for you. Including motors, ESC's, props and shafts.
Some of the things you might need to know before contacting anyone:
Brushed motor size just for reference.
Gear box ratio.
Approx RPM of both motor and prop with the brushed motor.
Prop size, diameter and pitch. (probably a very small pair of numbers molded on one blade).
Battery type and voltage you wish to use.
And just to be sure, you do know that brushless have their own unique speed controllers? You can not use a brushed ESC on a brushless motor and vice versa.
One thing to add, brushless are shown with a 'KV' value (pronounced Kay Vee). This is the rpm per volt you can expect when unloaded. IOW, a KV of 1500 is 1500 rpm for each volt of battery. So a 7.2 pack would deliver around 10,800 rpm at the shaft.
Don't worry, it gets more confusing before it gets better.
