Plenty of people use their smartphone to capture photos and videos everywhere they go. To capture clear shots under water, you are going to need a decent underwater housing. The GDome Mobile Pro V2 can help. It has a 7-inch dome port and lets you capture split shots like a pro. It has a side sealing system, a built-in hand grip, and an air gap to prevent accidental touches.
You can use this underwater housing with 360-dgree cameras, 3rd party filters and lenses, and phones up to 7.3 inches. You can find out more about it here.