I use a plastic paint roller tray and a soft deck brush on a pole. Basically like painting the bottom. I can get most of the hull this way. Then at the end of the season I will don my glasses, gloves, etc. and crawl underneath and use a hand brush to get the areas I can't get from above. Unfortunately, it's tough not to get any on the trailer but I keep the hose close by to continuously rinse it. I have an aluminum trailer with galvanized cross beams. It stains the galvanized steal. So I have a spray can of cold galvanizing paint to retouch.
TIP: don't get ON/OFF on the chrome logos on the boat. It will stain the chrome.