The answer is it depends. And there is not even consensus among veterinarians about what diet is best for this situation. It depends on the cause of the urethral obstruction – if it was struvite crystals or stone, or a calcium oxalate stone, or a mucus plug causing the obstruction. It depends on chemical analysis of the human grade fish you would like to feed.
Struvite crystals may be fairly easily manageable with diet. Calcium oxalate, much less so. Cats with an obstruction caused by other causes may not benefit from any particular diet formulated for "crystals".
The urinary diets are, in my opinion, the safest bet. They are restricted in magnesium and phosphorus, which are the components of struvite crystals (also known as magnesium ammonium phosphate). They are also calcium restricted to prevent calcium oxalate stones. The goal is a 'neutral' urine pH which is not prone to struvites (which form in alkaline urine) nor oxalates (which form in acidic urine).
Your location will dictate which urinary diets are available - but there are a number of brands available, and you may be able to order other brands online if they are not available at your vets. These diets are more expensive than normal cat foods, and do usually require a prescription.
The most important thing, in my opinion, is keeping the urine as dilute as possible. Crystals form because the urine is supersaturated in the components that form those crystals. If the urine is kept more dilute, then the crystals will less easily form. The best way to promote dilute urine production is with increased water intake. Dry cat food has very little water, so is much less preferable in this regard. Putting urethral obstruction cats on a predominantly or even solely wet food diet can help to promote a more dilute urine environment. Also offer plenty of water sources, perhaps a water fountain if your cat is amenable.
If you would prefer to avoid the urinary diets, then please speak to a veterinary nutritionist who can formulate a balanced home-cooked diet that is suitable for prevention of lower urinary tract disease. There are so many home-prepared diets that are not balanced for cats' nutritional needs.
At the end of the day, there is only so much we can control in urethral obstruction patients. But diet is one of the things we can easily control, and does seem to play a major role. I often see several urethral obstruction patients in a week, and in my experience it is uncommon to have an obstruction in a cat that has already for some time been on a strict urinary diet. So in that respect, these diets do seem to really help.