Luke,
I am not sure how much quality you need, but the concensus seems to be that of the 3, the W80 is the worst of the lot. That's not to say it's bad...it's just not as good as the other two.
The 10MP W60 seems to have a slight edge on the other two in terms of image quality, it's definitely better than the W80. The W90 is better than the W80 but I'm not sure how it compares to the W60...however, it's even tougher, being shockproof, coldproof, heatproof, mudproof and waterproof, and has a way cool Digital Microscope mode.
There is a very informative test from Q2 2009 on a group of waterproof cameras. It compares the W60 and W80 directly. The W80 didn't do so well, while the W60 got the runner-up nomination from the group.
'Waterproof' Camera Group Test (Q2 2009) Review: 1. Introduction: Digital Photography Review
The W90 has some additional features, and you can google for sample images and such. Here's the DPreview blurb on it:
Pentax unveils Optio W90 rugged compact camera: Digital Photography Review
Ned Bunnell also talks about the W90 and its Digital Microscope mode in his blog:
NED BUNNELL: First Weekend with the W90 NED BUNNELL: W90's Digital Microscope Mode
Also, if all 3 of those cameras are too much money, you could look into the quite-budget-minded WS80. It's a much cheaper camera, and although it lacks a lot of the features of the W60/80/90, it still makes good photos. The only limitation I see is if you take it surfing, it's 5ft waterproof rating may not be enough.
Pentax unveils Optio WS80 waterproof camera: Digital Photography Review
The WS80 is about a third the price of any of the others, and it takes decent pictures. My fiancee has one and it does alright. Not as good as the 60 or 90.
So my short, informed but not necessarily expert opinion is thus:
If you need a good waterproof digital with good IQ above all else, W60. If the digital microscope features and additional strengthening are a good thing for you, get the W90.