I remember being given a card when I did some BTCV (conservation) work back when I was in college, more specifically as we did a project clearing up a long neglected pool (as in a big pond) in a wooded area and build a boardwork over it to make it somewhere nice to go. We dredged up all kinds of stuff and dead wildlife like frogs and we had to be aware of the risk of any breaks in our skin and the water (and whatever lurked therein) getting in and were told to produce our Weils Disease card if we got flu like symptoms soon afterwards.
Personally the though of swimming in something likes the Thames makes me shudder with the thought of what's in it. Similarly watching the swimmers out in the quays and canals round here - in such an urban area I dread to think what lurks.
You might want to check with kayaking in case they intend to teach you rolls and such or playing games such as making a multi-kayak raft and walking over them (inevitably someone has to fall in!). We used to do that when river kayaking (though nice cleaner Shropshire/Welsh rivers) - quite different trying to roll in a current than in a swimming pool, especially if you don't make it round and have to abort from underneath.
Word to the wise...while kayaking, stay well clear of swans as my friends found out to their cost!