Well without HR, Strava has really no way to tell how easy or hard the activity was for you… here's how it works:

https://support.strava.com/hc/en-us/articles/360000197364-Relative-Effort
 

You can set the intensity manually if you want:

https://support.strava.com/hc/en-us/articles/360032535512-Perceived-Exertion

Some wearable heart rate monitors are compatible with swimming and can be paired or your activity synced with Strava if you want to invest. They typically have some onboard memory since RF doesn't transmit through water. 

Hi ​@fun, thanks for your thoughts! Based on the tips and advice provided so far by ​@Jana_S and ​@arc88, this post will be converted to a conversation. If there’s any concerns around that, let us know and we can take another look. Thanks!