1. fun and good exercise swimming is a very useful skill.

问题补充：

如上

答案：

()being fun and good exercise,swimming is a very useful skill

答案：It

这里是独立主格结构，相当于一个状语从句，这里就等于Because/Since it is fun and good exercise, swimming is a very useful skil.

意为：游泳很有乐趣，是个不错的运动，所以说游泳是项很有用的技能。