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There are several options here--everything depends on what you are willing to pay, how you want your boat to function, and how you want it to look. My personal opinion is that the OEM RPS is hard to beat. It's great-looking, comfortable, sturdy, and it utilizes cockpit space very efficiently (an important factor on a 17' boat). Specialty Marine is selling RPS units for <$600.00 these days
http://www.specialtymarine.com/Boston-Whaler-Cushions-amp-Seating/Classic-Boston-Whaler-seating-1958-1999-c1/, and used ones pop up on ebay and craigslist fairly often. Of course, if you factor in shipping to Hawaii, you're looking at a fairly hefty price tag either way.
There are problems inherent with a cooler helm seat. First, If you want a cooler that will withstand the abuse of a couple of grown men bouncing up and down on it repeatedly with the kind of force that a Montauk in open water can generate, you're going to have to go with a high-end unit such as a Yeti, which will cost almost as much as a new RPS.
Second, if you have a cooler under the helm seat, you will have to find another place for the fuel tank(s). This means either locating them under the console, which limits storage and brings up safety issues (such as ventilation and battery location), or leaving them in the cockpit, which will eat up fishing/diving room and move a lot of weight aft.
Assuming that you are not overly concerned about maintaining the classic Whaler look, a leaning post might be the way to go. It would allow you to keep the fuel positioned and ventilated optimally while saving weight, cockpit space, and a few bucks.