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More specifically: can I order the Broom of Flying to dive into the sea so a sinking PC (several meters underwater) can grab it, then order it to fly out and back to me, saving the PC? I figured, creatures with literal wings of course couldn't move through water that easily, but a magically propelled broom wouldn't be aware of the substance it is surrounded with, would it?

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Flying movement is intended for use in the air

Flying Movement is described on p 196 PHB

Flying creatures enjoy many benefits of mobility, but they must also deal with the danger of falling. If a flying creature is knocked prone, has its speed reduced to 0, or is otherwise deprived of the ability to move, the creature falls, unless it has the ability to hover or it is being held aloft by magic, such as by the fly spell.

Underwater, there is no danger of falling. When you are moving under water and stop moving, you at worst sink slowly. So like in common language usage, you can use a flying speed to move through air. To move through water, you need a swim speed, or you need to pay double movement, which normally refers to your walking speed. It would be up to your DM to allow using the flying speed of the broom to swim like that spending 2 feet of movement for each foot traveled.

We can also look at example monsters. For example, the Young Bronze Dragon has a fly speed of 80 ft. and a Swim speed of 40 ft. If it could use its fly speed to swim, it would not need a swim speed.

Of course, for the sake of dramatic action, you as DM can always allow such dive-bombing. You are given explicit license for this by the game (DMG p. 4):

The D&D rules help you and the other players have a good time, but the rules aren't in charge. You're the DM, and you are in charge of the game.

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    \$\begingroup\$ Having a swim speed also means you can make melee attacks without disadvantage (5esrd.com/gamemastering/underwater-combat). So even if you allow fly speed to work as your speed for the normal rule of movement costing an extra foot per foot underwater without a swim speed, Fly 80 / Swim 40 still matters for things other than movement. (And for simplicity to avoid DMs having to figure out how fly speeds interact with water.) But I'd guess the intent is that you can use your walk speed when swimming, since that usually just gets called your "speed". \$\endgroup\$ Commented Jul 7, 2023 at 21:41
  • \$\begingroup\$ @PeterCordes I agree with everything in your comment. I thought about bringing both these up, and it got bit long-winded for a secondary point when the main argument is based on the description of the flying speed. \$\endgroup\$ Commented Jul 7, 2023 at 22:00
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The broom has only one type of speed

Can something swim if it has only a flying speed? The rules assume that "your speed" by default is your walking speed, and that if you have other speeds they must be indicated as such - "your flying speed", "your swimming speed", etc. Thus when (PHB Chapter 8: Adventuring) says that:

Each foot of movement costs 1 extra foot (2 extra feet in difficult terrain) when you’re climbing, swimming, or crawling. You ignore this extra cost if you have a climbing speed and use it to climb or a swimming speed and use it to swim...

we understand that for something to swim (move under or through water) when it does not have a swim speed, it costs two feet of walking movement to move one foot swimming.

It is tempting to conclude from this that since the broom does not have a walking speed, that it then cannot swim: that is, only things with a walking speed can swim without a swim speed.

However, there are lots of creatures that have a flying speed, but a walking speed of zero (the flying sword, for example). If the interpretation above were correct, if things with only a flying speed could not move in water, it would mean that I could effectively and without a save paralyze such creatures simply by immersing them in water, whether in a watery sphere or a mundane drinking trough. This does not seem like a reasonable outcome, and makes me question any interpretation that says you have to be able to walk in order to swim without a swim speed.

Rather, consider what is written (in PHB Chapter 9: Combat) about Using Different Speeds:

If you have more than one speed, such as your walking speed and a flying speed, you can switch back and forth between your speeds during your move.

If you have more than one speed, you can switch back and forth. But if you don't, like the broom, then you can't. The broom has only a flying speed. It can't switch back and forth, so all of its movement has to be calculated from its flying speed. Understanding this, when we again read:

Each foot of movement costs 1 extra foot (2 extra feet in difficult terrain) when you’re climbing, swimming, or crawling...

we can interpret this such that each foot of movement swimming costs an extra foot of its flying movement. Whether or not the (nonsentient) broom is "aware of the substance it is surrounded with", it will cost it more movement to move through the water.

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