OK, without the cropping the water represents even more of the image. So the matrix meter is influenced more by it and tries to compensate. Notice on the backlit shots the water is reflecting more of the bright blue sky and on the front lit shots it looks more brown? That is why you were having to set plus exposure compensation on the backlit ones but on shots 3,4,5 that are front lit the water is darker as well as the bears being dark so the negative compensation worked for you.
In these backlit conditions you can get some pretty interesting shots particularly since the bears heads are blond. If you expose properly for the shadowed part of the bear the uninteresting BG will wash out and the fur will provide a bright outline where it is lit by the sun. You don't want to completely blow the highlights but if the detail washes out in the BG, all the better. I'd also try to flatten DOF a bit by shooting a larger aperture. Were you shooting your 300mm on these? Since they aren't in a particularly photogenic setting you may want to shoot and/or crop as tight as you can on them. Use your extender and flat DOF. As long as you get the bear's face sharp that is all you really care about. Let the rest of it fade into blur and/or over/underexposed lighting.
The water running off their faces is pretty cool. Did you try using a brush tool to get some detail in the eyes? Some of the LR experts will have to chime in on that. In PSE I'd do it with a copy layer. A vertical crop would eliminate even more of the BG on the face shots.
Based on how well you improved on the herons I'd say you'll get the bears down in another trip or two