Magnet boards are great memo boards. Here are a few coastal theme ideas & links to tutorials.
This magnet board is also a chalkboard, made with sheet metal and chalkboard spray paint. Seen here. You should be able to find sheet metal at a hardware store. Check the scraps and plumbing section, and bring a magnet to make sure you're buying the right one. You can mount a metal sheet directly to the wall, onto a piece of wood or slide it into a frame. To add a coastal theme, magnetize shells, beach rocks, driftwood or pretty seaglass pieces as shown below.
Seaglass magnets made by Dana of CereusArt.
There are so many ways you can personalize your sheet metal. Cover it with a great photograph as seen on Magna Memos, use a fun Fabric, scraps of wallpaper, Maps or line it with contact paper. Use spray paint to adhere whatever you choose.
And think of all the frame choices. You could create a Shell Frame with your beach finds as seen at Beach Grass Cottage. Another idea. Use an Old Window Frame and replace the panes of glass with sheet metal. You can also make magnet boards with canvas by gluing a sheet of metal to the back. Or try magnetic paint.
This is Ikea's Spontan Magnet Board decoupaged with Vintage Maps from eBay. Read more on Apartment Therapy: How to Make a Magnet Map Board.
Ikea also sells little Magnet Containers. People usually use them for spice storage, but they make the coolest magnets with little beach scenes in them. Take these Mini Fridge Magnets made from round aluminum favor tins as an inspiration.
And a magnetic memo board made out of a vintage tin tray. Via Pinterest. Buttons were covered with a white and blue striped ticking to make matching magnets. You could make something similar using a baking sheet.