How to ride the wave for complete bronze trophies?

When you get to the 3rd (2nd?) settlement, click on the construction button. Under the collection of 'decorations' you can do (the ones that give you influence for building them), there should be one with an umbrella called something like "Surfer Camp" with a cost of 1,000+ gold. Build this. (the only location(s) it is allowed to be placed is near water). After you build it, click on it. A surfer will sail down the waterway (river). The first time you do this, you will have completed the trophy. (though you can do it more than once for amusement if you like)