I stayed up late last night playing around in Unreal to find a good visual style. I don't think I've got it yet, but I reckon I'm close. It amazed me just how many tools Unreal gives you to make things look good in an incredibly easy way. Of course, the copious amounts of plugins and assets I downloaded from the store make that selection much wider.
Today, I just focused on getting the core mechanics down. After double (or triple) checking the documentation for the physics water system, I managed to add a body of water very easily. I then added a buoyancy component to a cube and made it float. That part really is that easy.
I then tried to make it so that the boxes would sink under the water when the player jumps on them. That took a lot longer. Meanwhile, I was also trying to figure out how to implement my tether idea. I'm not likely to make this one a projectile system, much less bounce around or wrap around corners, because it took so much effort just to adapt a simple grappling hook I'd made before to pull the other way.
Having said that, I was able to get the core game made within a day, so nothing was really that complicated despite how much I say I struggled with it. All that's left to do now is make some kind of visual effect for landing in the water, (I'm thinking of making a splash effect on a UI element and triggering that when the player passes a collision box) design some actual levels to explore, and polish up the visual and mechanical details. I might even get the full game done by August!
Here's what I have so far: