▲swim 作名词主要用在常见的短语 go for a swim 和 have a swim 里，但也有以 swim 为中心语的名词短语充当主语的情况。注意由于 swim 是可数名词，不能直接放在主语位置，须加上限定语（如冠词 a 和 the 等）或变为复数形式。所以题主给的 Swim is fun 是错误的，应当改为 A swim is fun 或The swim is fun.

▲以下例句摘自语料库和英国报刊：

1. So a swim and a cool down in the afternoon is the order of the day. 所以下午游个泳降降温是每日惯例。

2. Maybe a swim in this scallop-shaped pool will help to get rid of some of this excess baggage. 或许在这个扇贝形的泳池里游个泳会让你摆脱这一重负。

3. The swim has also raised money for Tim's Operation Raleigh expedition toChilein September. 那场游泳也为提姆九月份去智利参加的雷励行动探险筹集了资金。

4. Walliams' gruelling 22-mile swim raised over £1 million for Sport Relief. 让沃廉姆斯精疲力尽的22英里游泳为体育救济会筹集了一百多万英镑。

5. His morning swims at the Queen Mother Leisure Centre in Victoria are a vital part of his training. 他在维多利亚的王太后休闲娱乐中心的晨泳是他训练里很重要的一部分。