I have purchased and iPhone 11 ,can I use it to do underwater photography in both my pool and the sea ? If I can then for how long can I use it for under water ?

I would not take it into the water at all, and definitely not salty seawater!

About splash, water, and dust resistance of iPhone 7 and later - Apple Support

"were tested under controlled laboratory conditions:" which means not real-life conditions such as breaking waves, etc.

"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

No. https://support.apple.com/en-us/HT207043