Let's say you lose 2 liters of water per day. To replace 2 liters of water by eating fish, you need to eat, for example, 2,500 g of cooked Pacific cod, which, besides water, also contains 500 g of protein and 3.4 g of sodium.
Consuming 3.4 g of sodium (8.5 g of salt) per day is not that much; some people consume it on a daily basis. I calculated (from here) that this amount of salt can result in about 240 mL of water loss by urine.
The bigger concern is protein, which is broken down into urea, which is excreted through the kidneys and drags some water with it in a similar way as sodium does.
1 gram of protein yields the amount of urea that needs 8 mL of urine to be excreted (see the source below). Consuming 500 g of protein from fish would therefore result in 4 liters of urine. So, consuming 2 liters of water by eating 2,500 g of fish leaves you with 2 liters of negative water balance solely due to urea.
So, if my calculation is roughly correct, eating only fish will make you even more dehydrated than eating and drinking nothing.
Urea, a major end product of metabolism of dietary proteins and amino
acids, requires water for excretion by the kidneys. Renal excretion of
1 g of urea nitrogen (2.2 g of urea) requires 40 to 60 mL of water.
Thus, if a person consumes 63 g of protein the volume of water
required increases by 0.4 to 0.6 L/day above the basal osmolar
excretory requirement of 0.5 and 0.75 L/day in younger and older
To add, I found this on Seeker.com; not sure how reliable it is, but it sounds plausible:
Drink the aqueous fluid found along the spine [cerebrospinal fluid] and in the eyes of large
fish. Carefully cut the fish in half to get the fluid along the spine
and suck the eye.