There is a lot of tourism money that goes untracked, but there's probably also a lot of tourism spending that leaves the island (big ticket purchases like car rental, hotel, airline etc like we discussed), so I'd say it probably balances out and tourism is 20-21% of the economy.
Imagine Hawaii if we reduced that by 1%, 2%, 3% or even 5%?? Sure, locals would have less income but maybe no traffic? Roads lasting longer? Emptier beaches? Emptier hikes? Less stress?
Paul Brewbaker, economist who used to work for Bank of Hawaii, wrote a report that tourism spending has been flat since the 80's. Meaning that more and more tourists have been arriving annually but each tourist is spending less and less. So the state has been pounded by record numbers of arrivals every year but the revenue hasn't increased in 30-40 years.