Why does raw celery from a restaurant taste different than raw celery at home?
There could be two factors involved. First, the freshness of the celery. Restaurants, especially those that serve large amounts, are going through a lot of product. This means that you are more likely to get really fresh celery.

Next, storing in water is very likely an issue. I would suggest buying celery within a day or so of when you plan to use it. Wash and dry it thoroughly. Then trim and cut to your desired size. Then immediately place the celery in a plastic container or zipper bag and place in the refigerator until serving.

Each time you take it out for serving, remove the amount you plan to use and return the rest to the refrigerator. The 'leftover' celery should remain fresh and crisp for several days.