So, the key to this question is realizing that we're talking about parts of a whole. The whole, in this case, would be 100% of people. (The term 'percent' literally translates to 'per cent,' or 'per 100.') So the givens are stating that for every 100 people, 14 are in the water and 37 are on the sand. So, if the rest of that 100 are on the boardwalk...
100 = 14 + 37 + x
And then you should be able to solve for x from there.
Hope that helps!