My first thought (based on a literal interpretation) was...who the heck would take their pet goldfish out boating with them? Along the lines of "take your son boating".
Syntactically, the sentence is obviously a command with the implied subject "you".
Semantically (and phonetically), I interpreted the tagline as follows: You take a fish when out boating.
This interpretation made me think the ad was created to promote the sport fishing industry and all that goes along with it: boats, bait, lures, rods, bobbers, etc.
It wasn't until I googled the phrase and saw this image of a lure, which by the way, was not on the billboard, that I realized the billboard was advertising a type of lure that resembles a fish.