I'll try out people's levels after I finish tweaking mine if my internet connection remains steady.
Here are two underwater levels my sister and I have been working on. I mostly did the first, and she mostly did the second, though we worked on both together.
Haunted Kelp Forest:
1C27-0000-0060-65A8
Baby Blooper Track Trial:
FD65-0000-005B-B1D8
I like making water levels because the way Mario can move freely in water coupled with the changes to enemy behavior in water makes for some unique possibilities. I'll probably be making several more in the future so any input would be appreciated.
There's a MM player called Spanky. His levels are not hard but they are excellent and inventive. Give them a go if you need some ideas.