Water leaking through a knackered sender............
As suggested on here mate, sort all your water leaks out before panicking about headgaskets. What colour was the smoke? As said, it could have been steam, but that could have been from the water splashing on the block and exhaust, but obviously with you travelling, you only saw it at the back. Only real way to confirm its not HG is a compression test, but don't flap yet!