A small safety kit. This is something that gets overlooked by a lot of new owners, and something that is really useful to have on board.
Pretty much anything you can think of they'd like, but with the name of their boat on it. This may sound simple until you start thinking of the possibilities. A nice set of beer glasses with the name of their boat engraved on it. Shirts with the boat name stitched on the pocket, or maybe personalized crew style shirts. Kitchen or bath towels with the name embroidered on them. The list of things you can personalize goes on. This is a gift for boaters that'll be sure to stand out and be appreciated.
A towable tube. This is something that's fun for everyone and doesn't take any real skill from the recipient. The tube is simply hooked onto the back with a tow rope or harness, the riders hold on as the boat takes off and enjoy a fast wild ride. The tubes come in various sizes, holding one rider or up on a dozen. They come in all different colors and designs, so it'll be easy to find one that fits the personality of the person you're shopping for. This is something people are always excited to receive and they'll end up using it again and again.
Hopefully these ideas help take some of the stress out of finding something that will be fun, useful, and fit the personality of your friend. Getting something for their boat is a great idea because it's something you already know they love and care about. When looking for gifts for boaters keep in mind what they usually end up doing on the water, and then think about what you think would make that time better for them.