Really unique concept for a puzzle game - I've never seen anything like it before.
The way weight works wasn't intuitive to me. I assumed that I was either 'heavy enough to sink' or 'light enough to fall' and only after quite some time that it was possible to be at an 'equilibrium' weight where you neither sink nor fall. I also didn't immediately realize that treasures contribute to your weight, and had to do quite a bit of deliberate experimentation to figure out that treasures contribute 0.25 of what weights contribute to your total weight.
The somewhat awkward thing about treasures contributing a fraction of your weight means that those 'equilibrium weights' can only exist if you have a number of treasures which is a multiple of 4.
I also found a way to cheese the game (which it sounds like taximan also discovered a version of) - it's possible to drop a weight, become buoyant enough to start rising, and then grab the weight before it falls below your hitbox, causing you to fall again - if you repeat this over and over, you can 'swim' indefinitely. But due to what I noted above, this only works if the number of treasures you have is NOT a multiple of 4 - if it is, instead of switching between "rising" and "falling" you'll just switch between "rising" and "neutral" (and if you switch from "rising" to "neutral" weight while already rising, you'll keep rising - not sure whether that's intentional or a bug). It also seems exceptionally difficult/impossible to do if your weight is a multiple of 4 minus 1, since you get so little rising speed.
I got stuck on the final level (or what I assume is the final level because I went off the top of the screen instead of going to the next level) for quite some time until I had figured out the weight mechanic, since it seemed necessary to know how to make your weight neutral to get the top-right treasure, but I was indeed able to beat it without cheese.