at the bottom of the sea there is treasure. soon you'll get a message or letter in the mail from edo on cannon isle. he will tell you that you can get a savage arm and then you can go buy it from him...
You don't really get through it you just get to the end of the part you can go to right now, but you have to come back several times to completely finish it.
My advice to you would be to get as much sand as possible, (they're in treasur chests on the ocean floor) and keep hacking away at it. Also, try to save the time you've got.