My trainer told me watermelon is 'the worst fruit' for me.

Well, watermelon is high on the glycemic index. It's in the 70-80 range, versus something like an apple which is around 40. However, the glycemic load is still pretty reasonable because watermelon is relatively low in carbohydrates. That said, even glycemic load isn't the be-all and end-all of the blood sugar debate. Watermelon is about 90% water and very low in fiber. Meaning it's basically analogous to drinking sugar water (albeit with a good amount of vitamins A & C). I know that I could eat about 10 times as much watermelon as I could apples.

I think watermelon would make a decent post-workout food, to stimulate insulin and rehydrate yourself; outside of that context I'd pick a different fruit. To me it sounds like decent advice.

That's just my opinion; I'm not an expert by any definition.

Not true, the glycemic level is actually only 3.

http://nutritiondata.self.com/facts/fruits-and-fruit-juices/2072/2

Uh yea I said the glycemic load was low, it's the glycemic index which is high. And I also said why I think the glycemic load is misleading on a food like watermelon which is so high in water content and so low in fiber.

A good alternative (if you like them) would be grapefruit. It has practically the same water and sugar content as a watermelon, but it's much lower on the glycemic index (25) and has a lot more fiber & micro-nutrients.