Does anyone have any experience or recommendations for getting travel insurance which will cover getting stranded due to delays/cancelled ferries (or trains)?
Hotels, pay to reroute, etc. It seems clear when it's a flight, less so a train/boat.
If you're catching trains, or internal ferries, to reach your international crossing, it's almost impossible IME. Most travel insurance will cover problems with your international leg, and maybe problems with the domestic part that cause you to miss the international leg, but if you have a lengthy domestic journey on the way back they will say "oh, we only cover you until you arrive in the UK".
I've been burnt by this when travelling home from the Netherlands to Orkney. The ferry to Orkney was cancelled due to weather, causing me to have to spend a few hundred quid on alternative arrangements.... but my insurance cover turned out to end at Harwich.
If you're staying entirely domestic then you may have more luck, but they usually insist that you have to stay two consecutive nights in the same commercial accomodation - i.e. if you are going to one place, staying, and returning, you might have some cover, but if you're vistiing family or travelling every day, you're out of luck.
Travel insurance products all seem to be predicated on "Every traveller is doing a trip to an international destination and then coming back from there, and the domestic part of the journey is trivial".