I had my phone submerged in water for about 15 seconds. Less than 3ft if water. It’s an IPhone 7 and is supposed to be water resistant to a certain extent. Now my screen shutters and is not working properly. What should I do?

You got water in the phone. You can try letting it dry out, powered off, for a few days. If that doesn’t work the phone needs to be replaced.

Water damage is not covered by any warranty.

Apple specifically states do not submerge the device.

Water resistant does not mean waterproof.

https://support.apple.com/en-us/HT207043

To prevent liquid damage, avoid these:

• Swimming or bathing with your iPhone
• Exposing your iPhone to pressurized water or high velocity water, such as when showering, water skiing, wake boarding, surfing, jet skiing, and so on
• Using your iPhone in a sauna or steam room
• Intentionally submerging your iPhone in water
• Operating your iPhone outside the suggested temperature ranges or in extremely humid conditions
• Dropping your iPhone or subjecting it to other impacts
• Disassembling your iPhone, including removing screws

Read this: Understanding Water Resistance

That's not what the water resistance rating mean.

Water resistance to 1 Meter is barely splash resistant.

A device (watch, phone, etc.) should not be immersed in water at all unless it has a water resistance rating of at least 50 Meters.