Hello there
Hopefully I can answer your questions, and please feel free to ask:
=== HP Lines ===
The lines in the HP section is meant to help count the boxes (each line is between the 5th and 6th box). But in hinsight, I should have mentioned that the lines were for that. So I might make a small update to clarify that. But gaining more boxes as they level up could be a possibility if one wants a more "from zero to hero"-feel. But with how dungeon world works, having less than 10 HP is very dangerous, so I would advice against it.
=== Philosophy ===
I would like to explain the design philosophy before giving examples. I wanted magic to follow the same flow as other PbtA games: conversation to mechanic, supported by the fiction. The player describes the character’s actions and intent, and the GM evaluates how that would look like in the current fiction. This helps to break magic away from the D&D mentality of a spell list, making magic feel more like part of the conversation.
And being so freeform also allows the player and GM to shape the limits and flavour of magic, so that it fits the character and story/world the group is playing in. Again, all through the conversation and the fiction.
=== Examples ===
As mentioned, it starts with the conversation and fiction. So can someone with the Elemental Power: water control blood? Well, what role does blood have in the world? Normally the GM makes this call, but should still have a conversation with the player about it. Because if yes, water magic can control living blood, then that means going forward, that is part of the fiction.
I am not sure if you specifically want examples for Elemental Power: Water, but here goes.
- The player describes forming a rushing torrent of water, intending to knock the pirate overboard. The GM decides the pirate is distracted, giving the wizard a chance to do so and asks the player to roll.
- The player describes the character pushing all the water away from themselves, creating a large bubble of air in the lake so that their watery foes won't be able to reach them. The GM decides this seems a little over what normally could be done with Cast a Spell, but doesn't feel it fits as a ritual. So decides that the spell will take a moment to cast, giving the enemy a chance to attack or intervene.
- The player describes how they want to flood the whole city, raising the water level of the river. This the GM decides is beyond simply casting a spell, and would require a ritual. They decide that the character needs a Place of Power connected to the river and a roll to pull this of.
- The player describes their character forming condensed balls of water, firing a pressurized beam of water at their foe. The GM had described the orc as wanting to attack the wizard, so the GM reminds the player about this incoming attack. This is to make it clear that there is a danger to be dealt with here and now. The player doesn't feel to keen about getting cut down, so describes how their character tries to keep a safer distance from the orc by turning around and moving around the furniture. The GM asks the player to roll Defy Danger, and if it succeeds, it will open up for the character to do some magic without the risk of being attacked.
- The player describes the character dipping their staff in the water, creating a strange rippling portal with a vague image of a different shore so that they can escape the island. The GM feels like teleportation of this sort would need a ritual with the requirement that it will take some time (minutes) and will be unreliable due to the portal leading to an unknown place.
A lot of it is all about gut feeling rather than hard mechanics/limits. Which is why it is important to communicate with the rest of the group so peoples expectations are at the same level.