The whale stage looks neat,It plays pretty good although I'm not crazy about the jump physics.If your using states for player,enemies you could have this in the Mr.Ultra Jumping State.Take a look at alex kidd in shinobi world,this code is the function it uses.
ld hl,(_MrUltraYVelocityPtr)
ld de,68 ;Gravity
ld a,(_JoyPad)
and Button1
jr z,+
bit 7,h
jr z,+
ld a,(_MrUltraJumpAccent)
inc a
cp 16
jr z,+
ld (_MrUltraJumpAccent),a
ld de,20 ;Gravity
+:
add hl,de
ld a,h
cp 4
jr c,+
ld hl,$3ff
+:
ld (_MrUltraYVelocityPtr),hl
ret