Personally I say if it's your number one, book an excursion with a boat that will (virtually) guarantee you will see some--someone who has reviews that say they sailed around until they found some. Nobody can 100% guarantee it, but these guys are out day after day and can see patterns. We tried on our own to find some--Paradise Cove and Secret Beach repeatedly, all along the north shore, and we even did 4 islands--and never saw a single turtle
We were lucky enough to see 4 seals, and we were lucky enough to GO to Hawaii, so we were lucky enough!!!! But I say if it's your biggest thing, I'd try to have some paid help on your side!!
I'd also add--try early in your trip in case weather doesn't cooperate, you'd have other chances.