Until a few years ago there was a regular car ferry service from the UK to Hamburg, which was then curtailed to Cuxhaven, and then dropped altogether. So today the only sea crossings are to neighbour destinations, from where onward travel has to be by rail or road.
Probably the most useful route is the Stena Line service (www.stenaline.co.uk) which runs between Harwich and the Hook of Holland (two hours from Germany) with day-time and night-time crossings, and a journey time of six and a half hours. This service is well plugged in to the rail networks of both the UK and the Netherlands.
And for those who live in the northern parts of the UK, a good choice is to take the DFDS (www.dfdsseaways.co.uk) sailing from Newcastle to Amsterdam.
Otherwise there are a wide variety of short sea crossings and operators which focus on France and nearby Netherlands, from the Southeast. There’s a good one-stop guide to them all on www.aferry.co.uk.