There are two ways to load a boat onto a trailer from the water: by winch and by driving through the trailer ramp. While both are acceptable methods, there's a lot to be said about driving your boat onto the trailer. For the record, it's not as easy as you think.
You need to aim for the trailer dead center for the driving method to work. It's hard enough to do this in still water; doing it against a fierce current is a nightmare. In the latter's case, boat owners often set up their trailers facing the current so that the boat can be guided safely onto the trailer. Strong winds can also mess up your aim.
Well-maintained rollers will keep the boat firm and steady as it drives onto the trailer. Otherwise, the boat will simply slide off once you attempt to load it. If you don't check your rollers regularly, resort to the winch method instead. It will save you time, fuel, and the trouble of tantrums from other irate boaters.
Not for Beginners
Driving onto the trailer takes masterful control of the throttle and steering. If this is your first time owning a boat, it's better to get a feel for loading and unloading by the basics. Use a good winch to assist you in launching and retrieving your boat.