I donated blood 3 weeks ago, and went swimming the next day. I haven't donated in 10 years, so it never occured to me that I may actually feel like a snail swimming in molasses.

I swam again 4 days later and while I felt somewhat better, I still felt sluggish.

I started feeling "normal" in the pool a week later. Obviously I can't speak for you, but I would suspect you'll be fine, aside from maybe feeling a bit more tired than you might otherwise.