That does sound like tough trapping. I've never had to deal with tides. Down here I just put some backbreaker on a fake castor mound with a drowner rig in front of it and usually have one in the next day or two. I would imagine that it would be tough to get one to come up a trail way out of the water after some attractant. Do they have places where they regularly make castor mounds along the banks? I would assume if they did then they would be above the high water mark and you might could get a trap in around them. That's where a gland/castor based attractant will really pull.