Summer, at least for most boaters, is a time when you’ll want to be using your boat as much as possible—not maintaining it, or getting involved in major projects. Still, big jobs that involve paint, fiberglass, gelcoat, or other structural work require warm temperatures...
So summer can be the right time to do bottom painting or hull work, especially if you’re planning on hiring the work out. Boatyards and boatshops that are backed up in the spring are often looking for work during the doldrums of summer and that means you may be able to save money with discounted rates.
Otherwise, beyond basic engine maintenance (like checking the oil and filters and performing changes at the manufacturer recommended periods) summer is a season of maintaining the cleaning and polishing you’ve done during all the other season. Some of the basics:
Make sure you wash down your boat after every use. This will help brightwork, gelcoat and paint finishes last longer.
Rinse and flush your sanitation system with clean water from the dock a few times each season (it helps prevent odors from building up).
Scrub your bilges now and again. Performing little jobs like this as a matter of routine a piece at a time will help ensure you won’t have a huge mess on your hands come winter, when it’s time to put your boat away.