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You don't need pebbles to root anything. They may have had bacteria that caused the roots to rot. The water doesn't need to be very high, about one-fourth as high as the cutting, which only needs to be 4 or 5 inches. Don't forget to change the water every few days to prevent the buildup of bacteria. This article will help:

https://www.gardeningknowhow.com/houseplants/snake-plant/snake-plant-propagation.htm

You don't mention if you are changing the water? Change it every two or three days to prevent the buildup of bacteria. Also, some people let the cutting callous over before inserting it in water to help prevent rot.

Snake plants root easily in soil or sand. You can cut the leaf, let it callous for a few days, then insert it in succulent soil that has perlite added for more drainage.

This is a sign of overwatering. Does the container have drainage holes? Had you recently watered? If the soil is saturated and will not drain, go ahead and repot it in fresh soil for cactus plants. If you leave it in saturated soil, it likely will develop root rot and succumb. You don't need to bring it inside unless temperatures are dropping.

Here is more:

https://www.gardeningknowhow.com/houseplants/snake-plant/droopy-snake-plant-leaves.htm

https://www.gardeningknowhow.com/houseplants/snake-plant/transplanting-snake-plant.htm

https://www.gardeningknowhow.com/houseplants/snake-plant/snake-plant-care.htm

The best way to do this will be to take a single leaf and cut it into segments. Root these in soilless media, like a mix of 3 parts sphagnum moss and 1 part course sand. Here is an article that will help:

https://www.gardeningknowhow.com/houseplants/snake-plant/snake-plant-propagation.htm