Level design was well done and I really liked the music choice for the levels, but where I felt the game really shined was in its aesthetics. Sometimes people will use custom sprites that look nice but can look disjointed paired with vanilla ones, but you did a good job maintaining a pleasant, consistent look throughout.
One little bug I found was that the 1-up mushrooms in the bonus game would fall throw the pile of blocks in the lower left corner and travel beneath the ground.
But aside from that, this was a great demo and looking forward to playing more of it!
I see you fixed the issues I mentioned in the Hack Testing forum so that's great! The hack it's pretty good. However I had to stop playing in Keelhaul Beach 2 due to an issue that makes getting the secret exit impossible:
While you fixed the cutoff, you forgot to change Mario's initial position in the sublevel, it's an easy fix though:
Also, some minor issues related to tile priority with vanilla dirt graphics like these. It's an easy fix too, just put the priority off in the tile editor where you can see the map16 pages:
In the last screenshot the green fish is behind those vanilla dirt tiles mentioned earlier:
If you fix these issues, the hack will be even better, because I enjoyed the hack. Good luck!