I'm going to take a different approach to this question than most probably would and suggest building an EATS Road.
EATS, standing for Etho's Advanced Transportation System, is a boat-based road system. It was developed by YouTube user EthosLab, who is well known on the Minecraft Forums for building amazing mechanisms.
The basic principle of an EATS Road is simple transportation between two destinations. After building the system, all you have to do is jump into a boat and the boat will automatically launch and send you on your way.
Despite what you may think, EATS Roads are completely automated. You don't have to steer the boats at all; all you do is jump in and the road will take you from point A to point B. Also, advanced EATS Roads have boat equalizing systems: when you arrive in one dock, it will dispatch another boat back to ensure both destinations will always have boats.
The main reason EATS Roads are effective is because, unlike minecarts, boats are not dependant of tracks of any kind; their paths can be easily altered by adjusting currents of water.
Assuming your islands are fairly far away, building a nice EATS Road may be the most efficient mode of transportation, as it's faster than walking, and you don't have to do anything while in the boat. Also, by building an EATS Road, you can be using boats without "simply [using] a boat". :)
Here is part 1 of a EATS Road tutorial by YouTube user scjoiner.
Also, here is a modified EATS Road demonstration by YouTube user Eyozix.