Depending on the size of your rig, a padded collapsible drink cooler works well. As Sharky mentioned, instead of water, wrap it in a wet towel.
Yes, a rinse bucket is ideal but as long as you keep the housing wet/damp, the saltwater won't dry. You can then give it a good freshwater soaking when you get back to land.
FWIW, I always use a wet towel and never a rinse tank. Too much damage can happen if you leave it in a rinse tank, especially if there are several rigs using it. Scratched lens ports, broken/cut sync cords, housing clamps popped open. Even the vibration of the boat through the water can loosen screws.