Fun? Maybe.
Low ROI? Definitely.
Heli rides start around $300. You're better off driving around and talking to locals.
For a fantastic semi-submerged (not a true sub) yellow boat tour where you'll see lots of sand and the occasional starfish the sub is perfect.
If underwater critters are your...
Been hitting the west side of Maui every year since the late 80s.
Which area will you be located and what is important to you?
Tons of DOs with two suggested DON'Ts.
* Don't waste your money on a submarine or heli trip.