The Flysky-i6 is a versatile transmitter, with lots of controls. This means you can get a bit confused if you start playing with them. The user manual click here is not very comprehensive, so below are a few tips. A lot of them are for people flying drones with this transmitter, which is a lot more complicated than our boats with just two channels (sheets and rudder).
This link is specifically for boats, not drones; it shows you some of the useful things you can adjust in the controls menus. Probably not for the beginner, though it shows you how to get in and out of the menu options clearly:
This video shows how you can control the range and speed of sheeting in and out ("throttle" is drone-speak for sheeting, and "power" is how far out the sheets are eased):
trim v. subtrim explained: