@xoiceox
I dont believe it is a MAC vs PC issue as I have a Mac Book Pro, and a g5 with a 23 inch cinema display and the pics of the girl are the issue. As far as the lighting is concerned, my question to you:
1) Was the 580 on camera?
2) Were you using auto-WB(never use auto wb)
Photographers are master of lighting, not cameras. Therefore, to be a great photographer you must understand the lighting... We are all students of this.
Apart from that, my thoughts reside with the poses, expression and cropping. Some thoughts are technical, others are personal opinion.
in image 1: Would rather see the model in shallower water so her hands arent chopped off. Hands are a sign of femininity and in a beach shoot they are integral.
in image 2: the models hand is gripping her hair, her expression is kind of cheesy (i know, its totally a personal opinion, but still thought I would share... Just feels like a senior portrait look in a bikini)
in image 3: Cropped of feet
image 4 looks the same as 3 but reversed...
image 5: major tan lines! Burn tool?
image 6: I agree with the others. Bikini shots are to exemplify the female figure, both curves and angles. This doesnt portray either as the model is too submerged in the water.
images 7: My fav, but a very harsh shadow on the right side (our left) of nose. A fill light or reflector would be useful here.
Again, these are just my thoughts, I hope I dont offend you by them.