Here's my take:

Row = The effort you exert on the motion of your life. Don't just float around, go get what you want.

Boat = Your life. It's got a rudder, choose a direction. If you don't grab hold of it, you just go in circles.

Row Your Boat = have a purpose-driven life. What do you want? Yes, it's a tough question, because you actually can have whatever it is.

Gently Down the stream = DOWN stream. go with the flow, be gentle, don't force it, no rush, no race (but there are losers). The Universe loves you and wants you to be happy. It's willing to work with you.

Merrily = HAVE FUN - Take it easy, allow for pleasure, don't be so serious. Relax.

Life is but a dream = Take it literally, yeah just like the sleeping, dreaming, fantastical, make-it-up-yourself, crazy crazy things happening... because it is. Yes, Really. So just don't worry because it turns out the same whether you worry or not. Worry is where all that energy you don't think have to do what you really want to do is going. And the worry is actually worse than what you are worrying about.