Hi Howard, if you are going to think about some sort of protective electric 'fencing' I have some experiences which might be of help.
We had extensive damage in the garden due to rabbits and badgers, I've no experience with otters or dogs but there are probably some cross over problems. I bought my gear from Agrisellex.co.uk in 2011 and it's still going strong. Their web site has a lot of good info on it so I would have a look at that to start with. I have had some equipment from a local farm supplies store as well, such as the electric fencing twine or posts but some of the stuff is aimed at cattle etc. which is probably overkill!
Look at the rabbit equipment section for the 12v battery energisers. They run off car batteries very well although leisure batteries are theoretically better. These are very portable and can be set up quickly when intruders are discovered.
I've used the rabbit netting which is quick to set up and comes with posts attached, although the posts are quite widely spaced and the net can sag a bit between them. I have found that extra posts are needed, especially if you need to go around corners. You don't need to have it switched on all the time, badgers are nocturnal, not sure about otters. After a few days the critters get to know what's coming and stay away from it so you can leave the net up but not switched on. With badgers you can see where they trample it down when breaking in and then switch it on again.
The other alternative is wire strands or electric fencing twine, probably at two different heights around the pond. You may have to experiment with heights if you can't find recommendations on line. A couple of wires around the pond edge might help to put off herons as well. I think they prefer to wade into water rather than landing on it. The wire will be less visually obtrusive but then could be left up semi-permanently.
With both the netting and twine you need to keep it clear of vegetation otherwise it will earth the current. You will hear a clicking and in the dark you might see small sparks.
I think this is a humain way of dealing with intruders and could be successful if you are prepared to experiment a bit. A camera might be helpful with setting up if you have any idea where they are entering the water to get an idea of the best height of the wire. Our cats soon got the hang of it and learned to jump over the rabbit netting but badgers are too ponderous to try that I think. I wouldn't think otters are great jumpers either. Dogs might be able to jump over but would probably not risk the learning process.
I hope some of the above is helpful. Look on it as a Challenge!!
