I love your idea, fisherman - very romantic - but here's a couple of suggestions:
-source a diamond ring at a reputable local shop in your home country and bring it with you (keep it securely on your person at all times!). This saves you a) from being potentially ripped off in Bali, and b) of the stress of finding something appropriate when you should just be enjoying your holiday.
-buy a ceremonial/symbolic ring to propose with - something cheap (personally if I were to propose to a girl, I think I would use one of those edible ring sweets you get
). This saves you the worry of transporting something valuable on your person through customs / around Bali, and also then your other half can choose her own
ring, either in Bali or at home, depending on whether you find something suitable (and there is no stress/rush to find something, you can do it at your leisure).
Just a couple of ideas... but best of luck and many happy returns!