I am going to warn you... this post is completely and utterly random... I guess that's what you get when you use "watermelon swim" and "watermelon bike" as search terms (and why the heck I got hits for both of these and not for funfetti and running is really beyond me!).
We'll start with watermelon and swim because I like swimming more than cycling. I came across this blog entry that compares lake swimming and watermelon juice. According to the author, both are refreshing, reminders of summer, and way better cold. Personally, I prefer to swim in water that's not too cold... but not too warm... I guess I need the "baby bear" swim. However, the Watermelon Mint Cooler sounds intriguing, although I should not be trusted with anything sharper than a butter knife and all of that dicing is a little too time intensive.
So what does the cycling world have to offer for watermelon metaphors? Nothing, but they do have a "good" remedy for cold sores: eat as much watermelon as you can, go for a 2-5 hour bike ride, and eat more watermelon when you come back. I'm sorry I can't be more specific... it doesn't seem like they controlled their variables very well... so you might just end up getting really sick from eating too much watermelon or really tired from biking for too long.
And, to conclude this randomness, I'll leave you with something that has nothing to do with swimming or biking, but everything to do with "efficient" use of time and cake:
blog where I found it points out) vegan!