What is the relative Covid-19 risk of outdoor swimming right now?
You can judge this for whatever it's worth. This abstract has not been peer reviewed. However it indicates that vast majority of Covid-19 transmission has happened indoors, not outdoors.
Indoor transmission of SARS-CoV-2
Hua Qian, Te Miao, Li LIU, Xiaohong Zheng, Danting Luo, Yuguo Li
This article is a preprint and has not been peer-reviewed
… It is essential to understand where and how SARS-CoV-2 is transmitted. Methods: Case reports were extracted from the local Municipal Health Commissions of 320 prefectural cities (municipalities) in China, not including Hubei province, between 4 January and 11 February 2020. We identified all outbreaks involving three or more cases and reviewed the major characteristics of the enclosed spaces in which the outbreaks were reported and associated indoor environmental issues. Results: Three hundred and eighteen outbreaks with three or more cases were identified, involving 1245 confirmed cases in 120 prefectural cities. We divided the venues in which the outbreaks occurred into six categories: homes, transport, food, entertainment, shopping, and miscellaneous. Among the identified outbreaks, 53.8% involved three cases, 26.4% involved four cases, and only 1.6% involved ten or more cases. Home outbreaks were the dominant category (254 of 318 outbreaks; 79.9%), followed by transport (108; 34.0%; note that many outbreaks involved more than one venue category). Most home outbreaks involved three to five cases. **We identified only a single outbreak in an outdoor environment, which involved two cases. **Conclusions: All identified outbreaks of three or more cases occurred in an indoor environment, which confirms that sharing indoor space is a major SARS-CoV-2 infection risk. https://www.medrxiv.org/content/10.1101/2020.04.04.20053058v1?fbclid=IwAR0CWY2-QjKbLDVN7irhoWixX7yVkzJkLeUog0WGFS-jpAv_mIRSiCuPry4
Another preliminary study found that the risk of “transmission from surfaces outdoors is lower during daylight” and under higher temperature and humidity conditions. “Sunlight destroys the virus quickly,” reads the briefing. https://www.yahoo.com/news/sunlight-destroys-coronavirus-very-quickly-new-government-tests-find-but-experts-say-pandemic-could-still-last-through-summer-200745675.html