Sorry can't come up with anything......why is the water heater in the kitchen in the first place? You need to keep access to it for maintenance reasons so you can't box it in.

Yes, if you did build a box, it would have to be removable, so it could be something lighter weight such as lattice or plastic grill.

A box would limit your accessibility to your sink area with drawer and cupboard. The yellow wall colour in that corner really draws attention. You need to camouflage that area so I suggest that you paint the corner the same colour as the water heater. That way, you won't notice it as much.

If that is a gas water heater (vs electric), that could be a fire hazard.

It's definitely gas so you would have to be careful building any walls around it as you need to provide combustion air. leelee's idea of the shower curtain seems as economical and attractive solution as I could come up with.

How about a folding privacy screen. One with feet or legs to allow air circulation from bottom to the open top. You only need to cover 3 sides. It will be easily removable for maintenance.

Move it out of the kitchen.

Another question you should be asking, no matter how extreme it may sound...
How can I prevent CO (odorless gas) poisoning from the gas appliances...particularly the water heater? It's location in this kitchen is questionable at best.

Be sure to have your water heater serviced by a qualified technician every year and install a battery-operated or battery back-up CO detector and check or replace the battery when you change the time on your clocks each spring and fall.

It is gas (see the vent) and I'd be moving that trash can away from it. If that little brown cab with the microwave on top can be moved, I'd do so. That might open up the space so that you could build a closet around the heater and vent, but you'll still run into the sink cab with it. Really tricky problem. Cheapest solution would be to paint the yellow wall and do something fun with your table to pull the attention away from the heater and toward the opposite side of the room.

I am certainly no expert, but as a landlord, I would never have a water heater in the kitchen as that seems very dangerous. I realize this is not the easy advice, but I would do some research and see if you can get the landlord to relocate it.