The interface design, more specifically, usually contains those type: the HUD, diegetic UI and Meta. Let me break down those elements in the scope of the Deep Sea game and how they behaves. First, let me talk about the diegetic UI, this could easily one of the the most welcoming diegetic in modern game world.
Is diegetic good? In a way yes, it provides more visual reasoning, and makes the game simply looks better and sophisticating. However, the drawbacks are also quite apparent. The diegetic UI got the player distracted easily, and the in-world position could change when attached to different type of world objects. And if designed poorly or used too much, the game interaction consistency will be break. Due to the player constant need to memorize different layout.
And the second thing is the traditional HUD, which in a way, is still the best interaction model in my eye. The reasoning behind that is that the traditional HUD barely change, so once the player learnt them, they will always know where to look for something. And the game UI could take advantage of as many screen space as needed.
The meta UI is in a way, a more hidden interface, it does not make obvious appearance that distract the player from what they are doing. More specifically, they provide timely feedback. The common meta interface in RPG game is a red arrow around the player to telling where the damage is coming from. Or a whole screen overlay to tell the player they are in a specific situation like being poisoned. I think the meta model is reasonable in a lot sense and provide the key information that the player need to know, which could has a lot of potential in game development.
One last thing I want to talk about interaction design is indeed the ease of use. An interaction model for the game is not considered good just because it’s nice looking and complex. A good approach to design the interface is to well considered the media you are going to present it on. Like on PC screen, on iPad, or even on iPhone.
If you put a more sparse interface that suited best on PC onto iPhone screen, the interface will soon appear in unreasonable place for it to be fingers friendly. And even just for the iPad or iPhone screen, if you place the UI buttons in the places where it doesn’t fit the best, the user will feel awkward when trying to control them.
From my perspective, a good interface design is dynamic, it means the user can do slight modification to meet their using habit the most. In PC game, it means allow the user to map the keys as they preferred. In mobile platform, it means provide reasonable sized UI, and let the user have the option to move the UI buttons around they they feel most comfortable.