This can be attained on the first act of Hydrocity Zone. Playing as 'Tails only' makes it easier as he is able to swim to a limited extent (press jump twice).
To access the Level Select Screen, highlight the 'NO SAVE' option (with Tails selected as your character) and hold X followed by holding Start.
Follow the route shown in the video to find the 3 boats required to unlock the achievement.