This is a great initiative. It is weird if you think about it. Why are our waterways in many cities so polluted and dangerous that you can't use them safely as a human(or any animal)? We should take more care about making the environment we live in safe and accessible for everyone.
I am pretty happy with the Synology network drive I use for self-hosting email. Synology has developed a business email suite which comes with 5 mailboxes. You can buy permanent additional mailboxes for the price of what you normally would pay yearly to big tech. The mailbox suite is fully featured with easy to setup anti spam measures and an intelligent spam filter you can train by forwarding spam and not spam messages to it.
I can certainly recommend it should you want to self-host from home.
A good addition to this list is https://postmarkapp.com/. They are meet EU law in regards to the GDPR. They don’t have a free tier for their relay service, but do have a nice free reporting tool for DMARC.
I have been using https://soverin.net. It has a privacy-first approach to email, while at the same time they are transparent in regards to its operation. They operate and are based in the Netherlands.
Amsterdam (and many other cities in the Netherlands) have swimmable parts. I'm coming from Groningen myself and there the government made a beach on one of the canals: https://www.visitgroningen.nl/nl/locaties/2159964091/stadsst...
In Amsterdam, you have the Amsterdam city swim, a yearly event where participants swim through the canals.