@Will_A
I've never dived that side, so take that into consideration..
It helps to understand the area. The Persian Gulf is almost an Inland sea. The water ebbs and flows through the Strait of Hormuz. It's also very shallow, especially around the coasts of the UAE, Qatar and Saudi where you struggle to get to 40m. Because of this, in the summer teh gulf heats up. 38C at the surface and all the way down to 40m is not uncommon. So in these shallow areas coral is scarce. The gulf is very saline because of evaporation and the desalination plants around its coast. Be prepared to add more weight
That said, we do get pelagics. Off the tip of teh Musandam peninsula (the enclave of Oman which is one side of the Strait) Sharks are seen. We know tiger, and bull sharks are around as they're seen from teh air, and the occasional hammerhead unfortunately is caught by fishermen - They're not spotted whilst diving though.
We do know there is a whale shark nursery somewhere in the gulf. There is an aggregation in the Qatar gas fields (maybe others too) but these fields are off limits to divers. That said whale sharks do come down both coasts on their way out to the Indian ocean ( dive the Musandam for 2 days each month and sometime see them 4 - 6 times a year)
The Iranian side is deeper, still not much more than 80m, but you would hopefully benefit from cooler water.
Do not expect large coral gardens, they do exist but are rare, more likely is soft coral mainly whip coral and anemones. Certainly around the strait of Hormuz there is a ton of life because of the nutrients coming from up from the continental shelf 20 mile to the SE. Also the water is deeper and there are a lot of strong currents cause by the water trying to fill and empty the gulf.
I'm not sure that will translate to Kish Island so much as it's that much further north. Kish also has a couple of dive ops on it, so I suggest the shallow areas will not be in the best state.
The water temps in April will be mild (sea 25-26C) The Air temps may be in the 30's and it's not unknown to have a few early days of 40+ we start to stay indoors from May - Sept). Also at that time, Sharmals (strong winds) blow down the gulf as the equator heats up .
The visibility can range from nice and blue 20m+ to green murk of less than 5 or 10m depending on time of year and currents etc.
This is the chart of the area, and A zoomable version is
here
If you're interested this is the short documentary about the whale sharks of Qatar
Hope this is useful