I think the snake looks like a Mangrove Water Snake (Enhydris bennettii) which is fairly common at Mai Po (apparently - I've never seen one!). Note the dark upperside and darkish underside with pale whitish stripe down the sides.
Mr Tai, herons invariably eat their prey whole (they do not have suitable bill/feet for breaking it into pieces). It is usually swallowed head-first, and I think it is likely to have been the same for this individual.
Nice observation John - it's always good to see some interesting behaviour such as this.
