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?

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.

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.