Also, along the lines of what Chuck said, and you probably already know this, but on land all these lenses are 1:1 magnification, so when you are as close as you can focus, the subject is the same size on the camera sensor as it is in real life. Underwater, this would be magnified equally for all the lenses due to refraction. (Hopefully I'm getting this right.) So the difference between them is basically how far you are from the subject at full magnification. If you're farther, then there's more water between the lens and the subject, which can be worse, but you also don't have to get as close and potentially scare away your subject. If your subject is moving, a longer lens has a narrower field of view, so it can be a little harder to find and track something.
Also the longer the focal length, generally the slower it is to focus, but this doesn't seem very different for the 60 and 85 as far as I can tell.
If you might upgrade to a full-frame camera, then you wouldn't be able to use the 85mm on it, because that is made specifically for crop-sensor cameras.
The smallest advantage of the 85mm is that it takes 52mm filters (and diopters, lens caps, etc), which you may already have since this is a common size.