The only machine you should be considering is the Longranger. For short flights you will fit 6 small to mid sized Pax - Not very comfortable but doable (6 up/5 no problem) and it does depend on size of them.
An L1 with the C30P conversion would be fine and most have this done- You wont temp out ever!
You would have a 500kg payload after fuel, pilot and floats with enough fuel to get one way on your 25Nm island hop.
Generally reliable, cheaper to operate than a AS350. Smoother (if well set up nothing is smoother). Not as fast as a squirrel but over short distances wont make any significant difference and keep in mind whatever you choose will, for the time being, be the only option so people will just get in it.
An air filter for the engine will pay for itself many times over regardless of the machine you choose stopping both salt and sand.
If the whole thing is a tax right-off or a small part of a larger money making exercise you couldn't go past the 407 (genuine 7 seater).