Finn goes through a lot of water. I'm talking a cereal bowl full of water per night, so probably about 32oz a day.

He gets 1/4 cup pellets, unlimited timothy, botanical, orchard grass hay, and about 1.5 cups veggies a day. He has a salt lick because it's in the shape of a carrot and it's cute. he doesn't really lick it, but there is some wear. His litter box was void of pee last night and now it's about 1/4 pee, 3/4 clean litter. The litter box is a small cat box measuring ~12"x18"

The water bowl is in his pen and the cats have access to his pen so they may be jumping in, pawing the water, drinking it, etc. 

I don't know if there is a set amount rabbits should be drinking, but they definitely drink more by pound, than cats & dog. If you are feeding good quality pellets and veggies, then a salt lick is completely unnecessary and could actually be causing a nutrient imbalance. Feeding more veggies will probably make him drink less. How much does he weigh?

I have 2 water bowls for my bonded pair, that hold probably 3-4 cups combined. They will usually be empty after a day and a half. They get about 6 cups of veggies per day and they are each 3 pounds. I have one rabbit who pees all the time in smaller amounts and one who seems to hold it until he has a big pee, so I guess that depends on what is normal with Finn.

I've read 50-100 ml/kg for rabbits, but really you want to be worried about changes not the actual amount. if he's consistently drinking a certain amount, that's what he needs.

i saw a post about how rabbits can need to drink up as much water as a 20 pound dog. however my sister's dwarves don't drink that much water in my opinion. my rabbit drinks a lot of water, but maybe a little less than your rabbit but she has peeing problem. as long as your rabbit doesn't have a urine problem or a kidney problem he should be fine. you could do a bloodwork test to see if there is a kidney problem, if you are worried. my rabbit does not have a kidney problem, but she has been exposed to ecunni.