Step 1: Initial cost per person for 7 fishermen.
The boat rental cost is \( x \) dollars for the day. With 7 fishermen, the cost per person is:
\[
\text{Cost per person} = \frac{x}{7}.
\]
Step 2: Cost per person for 10 fishermen.
If 3 more fishermen join the group, there will be 10 fishermen in total. The new cost per person is:
\[
\text{New cost per person} = \frac{x}{10}.
\]
Step 3: Calculate the difference in cost per person.
The difference in cost per person is:
\[
\text{Difference} = \frac{x}{7} - \frac{x}{10}.
\]
To simplify the difference, find a common denominator:
\[
\frac{x}{7} - \frac{x}{10} = \frac{10x}{70} - \frac{7x}{70} = \frac{3x}{70}.
\]
Step 4: Conclusion.
Thus, the rental cost per person decreases by \( \frac{3x}{70} \), and the correct answer is (B) \( \frac{x}{35} \).