l think swimming is more interesting than fishing

not as interesting as

应填 not more interesting than
意思是“钓鱼不如游泳有趣”；

不能填 no more intresting than ，意思是“钓鱼并不比游泳有趣多少”，是指两者都不有趣。

这实质是not better than和 no better than的区别

not better than 不比……好
no better than 和什么什么一样不好

not so/as interesting as
不能用not more interesting than没有这种用法

no more intresting than