It would quickly* rupture.

Five-hundred pounds of water is only about 8 cubic feet in volume, so the bag's weight capacity would be overloaded after merely an eighth of its volume capacity was reached — assuming it started empty, as any other items in it would help it reach its weight limit earlier. It would then rupture, scattering its (damp or soaked) contents across the Astral sea.

An objection might be raised: doesn't the bag only store things specifically put into it? No, the owner is not so in control. There's no verbiage limiting access to only intentionally stored items, so an open bag allows anything to pass into it. This means that opening it underwater would result in the water pouring in.

The space inside the bag is described as a continuous finite volume, not a set of pocket-spaces for each individual item put in it; therefore, putting/letting water into it would get water on any other items sitting inside.

_{* How quickly depends on the local water pressure, but just “quickly” is plenty of precision for our purposes.}

1. None of the named magic items are found in the index...

... annoying, but it seems only the categories of magic item (rings, rods, staffs, wondrous items etc.) are in the index, not the specifics. Instead of using the core books to search for specific items, I recommend using one of the SRD websites - I use 5thsrd.org to quickly search for individual rules.

2. By default, you can't buy most magic items

Default 5e assumes that "magic shops" pretty much won't exist, and even goes on to suggest that potions and the like be bought at alchemist's shops instead.

From the DMG, page 135:

Unless you decide your campaign works otherwise, most magic items are so rare that they aren't available for purchase.

and further:

If your campaign allows for trade in magic items, rarity can also help you set prices for them. As the DM, you determine the value of an individual magic item based on its rarity. Suggested values are provided in the Magic Item Rarity table.

The Bag of Holding is an uncommon item, therefore in the 101 - 500GP range. At this point, it is at your discretion.

Magic items in 5e

Unlike previous editions (particularly the 3/3.5/4 era) 5e is designed to depend much less on magic items. Together, the:

• attuning rules (which limit the number of powerful items any individual character can have),
• the expressly "lower magic" setting (which generally restricts the purchase - and sale! - of magic items),
• and their "bounded accuracy" (lower difficulty, ACs and progression gradients)

...ensure that high level 5e characters are powerful because of their raw skill and ability, not the plethora of magic items they happen to be decked out in. This was a specific design decision going back to the very first D&D Next playtests. (As a side note, having your party find that Bag of Holding in some dusty and forgotten attic is also more rewarding than just stopping by the local Bags of Holding R Us and grabbing a few!)