It's been my experience otters will be out during the day but like deer are more dawn and dusk liking and will also be out during the night time. Probably more as a reaction to human presence and trying to avoid it.

The key to trapping otters is finding the entry exit routes they are using to get to a body of water. Or when all else fails, set out as many darn traps as you can. They cover a lot of ground and usually don't stick around an area for too long. But they will come back... Patience helps!