That sunken ship level took me a few tries. This was really hard... The boss is also kinda tough, but once you get into the rhythm of hitting him you can beat it easily as long as you don't mess up.
After that, I get turned into tiger form and get warped back out on the shore. Instead of walking all the way back, there's a hidden Return to Town door at the dead end that you can activate by destroying a block that only the tiger form can destroy.
The tiger form can't really go anywhere except this underground cave, below the town and to the left. I found the 6th heart container here, and now I keep dying again trying to get through this... some of these attacks do like several hearts of damage to the tiger.
Made it to the ninja house... with like half a heart left lol. Tried again, and this time, I found this one enemy that seems to ALWAYS drop hearts or potions every time you kill it, so I just grinded this respawning enemy for a while before going through the rest of the cave.
The retro music in this ninja house place is so much better than the modern music... The boomerang is a lot of help at this part... but it's still really long and difficult.
Daimyo Dragon... hmm, I have no idea what the strategy for this boss is, other than grinding for tons of projectiles and just spamming them.