Building boats and navigating them would require colonies of large brain specimens; this approach would carry a big energy cost. Not to mention boat propulsion,

Building boats and navigating them would require colonies of large brain specimens; this approach would carry a big energy cost. Not to mention boat propulsion, especially if you wanted to sail without depending on weather. Instead of boats it seems more efficient to replace hands/legs with fins/tails. Tongue in cheek, but only half - natural selection works on a different scale and with different goals, and in that it seems pretty good.