To unlock World 10, beat Boss mode with either Stapy of Stafy in under 9:00. World 10 is a series of time attack levels that includes a new Boss. To face the new Boss, you must complete Level 10-1 with a time less than 0:45, Level 10-2 with a time less than 2:20, Level 10-3 with a time less than 3:50, Level 10-4 with a time less than 2:10, Level 10-5 with a time less than 2:50, and Level 10-6 with a time less than 2:30.
To easily gain pearls, in Hotcha Springs (Level 2-1), go to the part where there is a little rock below the door you just entered, little white fish to the top left corner of the stage that inflate when you touch them, and a line of fish on both sides of you. Start by going to one line of fish and using the spin attack to take out the line of them, then go to the other side and take out that line. Continue to repeat this, going from one side to the next, until you get as many pearls as desired.