They're fine and actually quite nice for an entry level dslr but if you want the camera for shooting fish mostly, they lack commander mode found in higher end nikons. Commander mode allows you to fire flashes remotely above the tank.
You can easily get a D80 for the same money but with many more features that work well with aqautic photography.
That's not to say you couldn't accomplish the same with a D40/60, it just means you'll have to buy extra gear.