Unless the flashlight is waterproof, no you can’t use flashlights under water.
Ways to Make Flashlight Waterproof
You can put your flashlight in a waterproof bag or a case and use it underwater. However, if you plan on diving or going a bit deep underwater, the pressure might actually break the LED bulb or the glass, so you actually want to make sure the flashlight itself is durable and sturdy enough.
However, it is simply better to just buy a flashlight that is specifically for diving and underwater use. We recommend this flashlight.