It took a little bit to get used to, but now I find it much easier than the swimming mechanics in Sunshine and 64. I like having the option to use a shell to swim faster, and being able to slow down with the Z button, so you are not going totally out of control like I was before a toad or something told me I could do that lol
There is one on Galaxy (a hungry luma galaxy) that you must swim a lot... however, I accidentally skipped all the work and got the star fast (you must take 5 pieces to create a launch star... however, I swam to the top to breath (I was getting drawn) and I saw more water above my head and a wall... so I performed a wall jump and I got the shell for the star without any effort. Then, I entered again to see what was in the bottom (as I went directly to the top to get air the first time) and then I saw ghosts, fishes, jelly fishes and more annoying stuff, along with the blue star pieces. I just exited it