Excessive urination (medical term polyuria [pah-lee-YOOR-ee-ah]; abbreviated as PU) is a sign of diabetes. It may also be a sign of other conditions. See the FDMB thread below for other reasons for dilute urine.
Animals with high blood glucose levels urinate frequently and in large amounts to rid their bodies of excess glucose. With cats that use a litter box, excessive urination is an early but often overlooked sign of diabetes. The clumps in the litter box of an unregulated diabetic cat can be very large, numerous, and sticky.
Excessive urination often is accompanied by excessive thirst (medical term polydipsia [pah-lee-DIP-see-uh]; abreviated as PD). Diabetic animals often drink incessantly because they are dehydrated from the cell-dehydrating effects of hyperglycemia, plus the effects of their bodies casting off the excess glucose through urination, taking hydration with it.
In multi-cat households, it may be difficult to determine which cat is urinating excessively, but other diabetic symptoms, such as lethargy, weight loss, neuropathy, and excessive thirst can help you identify which cat is involved. You also could place the cats for a period of time in separate rooms with separate litter boxes to help you identify the cat that is urinating excessively.
It is fairly common for diabetic cats to urinate and sometimes defecate outside of their litter boxes. This behavior is not necessarily a sign that the litter box is in the wrong place, or not clean, or that the cat was not able to "make it" to the box. Cats often will urinate and defecate inappropriately when they do not feel well.
Inappropriate urination often is self-correcting once the cat's blood glucose levels become regulated.
With all cats, diabetic or not, you should practice meticulous litter box hygiene. The link below has an excellent article on caring for a litter box.
The concentration (or lack of it) is determined by what's called a urine specific gravity test. The basis for comparison used is the specific gravity of water, which is 1.000.