A zoo is a great place to develop an eye for animal stuff I reckon... And there a great place for a relaxing wander around...
The crocs in #1 are great, good detail, wow thought it was a nice short DOF, no wonder, 2.8...
The second one is having trouble with the lighting which is throwing fairly dark shadows in areas where you want detail... The EXIF says 1:30pm and looks like it was a bright sunny day so the exposure on animals in light and shadows on their face is really tricky... Makes you lose eye detail and stuff... The background is awkward, the trick with zoo shots (I think) is making them look as natural an environment as you can, but you take what you can get eh?
The seal is a bewdy, try cropping the top part off that's different to the water and it won't distract the viewer from the cute seal...

Clipped his flipper but doesn't really matter as it's not a focal point... Oh, hang on, why not try cropping the top white background part off, AND the concrete footing at the bottom and make it more of a portrait?
Nice use of DOF in the flower shot, I'd maybe try a smidge of extra sharpening for the centre of the flower maybe? Great color... Try it with a much smaller aperture too and see what it comes up like, be a good experiment...
The last one, the waterfall shot is nice, the highlights in the foreground horizontal log are a smidge hot, and maybe the spot of light high up in the middle of the rock background maybe burn (darken) a smidge so it doesn't take your eye away from the waterfall and stuff maybe???
Enjoyed them
