Exiting a boat before your hand reappears causes your hand to disappear.

If you are riding in a boat and hop out while your hand is not on screen, it won't reappear. Items you are supposed to be holding aren't visible either. Tools like pickaxes still break blocks, however you are unable to hit entities with either your fist or a tool. You also cannot place blocks while your hand is missing. If you swap a block or item into your offhand, those won't be visible either. So far, I've only been able to fix the bug by exiting out of my world and rejoining.