Yeah, it was hard to keep absolutely stable on the sand. Any tips for something like this? Maybe get a bag to weight the tripod down straight or dig it in a little bit?
Sure, move back a bit or increase the surface area of your tripod feet. There was probably too much water in the sand which meant that your tripod was sinking as you were taking the shot. So make sure your tripod is locked up and move back to above the water line, shoot a bit wider and crop later or zoom to get the composition. If there are plenty of rocks aroung put flat ones under the tripod feet to distribute the weight over a bigger surface area which should stop it sinking. Digging the tripod in or adding weight would only help if it's the waves hitting the tripod legs that's causing the vibration, but even then I'd be more inclined to move back to a more stable position.
Rule number one for long exposures is you need a stable base, if you don't have that the image is screwed. By all means take a shot in the position you want but chimp it after and check for movement, that way you can check for movement and stabilise and re-shoot.