Fully waterproof camera flashes are uncommon outside dedicated underwater systems. Based on the answers, the more realistic options are:

• Use a weather-sealed speedlite rather than expecting true waterproofing. The Canon 600EX was noted as having improved dust and water resistance, including sealing around buttons, display areas, and the mounting foot when used with its sealing adapter.
• Put a standard flash in a waterproof enclosure or bag. Community suggestions included protective hard cases such as Otterbox-style setups, or adapting waterproof pouches/bags if you can still route a trigger connection.

So, for a Canon 5D, a weather-sealed Canon-compatible flash is the closest off-the-shelf answer, but it is not the same as being fully waterproof. If you expect heavy spray or real water exposure, a protective enclosure is the safer approach, and you’ll also need to plan how to trigger the flash from the camera.