Hi,
Sounds like a lovely project!
My first concern or hurdle would be to think about how you are going to get photos of an animal that probably barely stops moving, using just artificial / outside lights. It may surprise you to know that despite having strong artificial light, it really is no substitute for daylight and when you take an image thinking it's well lit, your camera will be telling you it's way too dark. Your eyes are much more clever than the camera.
I don't want to put you off, completely the opposite - to avoid the pitfalls and get some great shots!
Can you let us know the type of gear you are thinking of using as they may help with advice?
My main experience of night photos is my back garden badgers and foxes but I normally use flash unless it's early in the evening.
As for position of lights - not pointing right at the camera is a good start. I would personally prefer to find an area they may linger at / fish from / eat at and light that. Maybe from both sides at an angle of 45 degrees or so?
I'm no expert - just throwing out some ideas.
It is possible to get images at dusk without flash like this next one for instance but it does depend what time they appear and the time you have to wait! -
Using flash will help alot IF the otters are happy with it - this badger shot is using one off camera flash ( I'm in the market for my second flash to improve these further)
You mention you arent a photographer so won't go too into it until I hear what your kit / experience is and maybe a bit more info on the otter habits?
Good luck!
Oh....and I'm very jealous!!!
TobyH