I think you have two problems:
1. You haven't used は to contrast anything. It is just used as a typical topic marker in the two sentences, with the topic changing from May to August.
2. Your adjectives need to be conjugated, or you need to use different verbs.
Does that give you a hint or two?
Edit: Actually, I'm trying to come up with a sentence that uses は as a contrastive marker for the example you gave, and I think any example will end up being a bit complicated.
On the right track with the verbs/adjectives.
The use of は is still just used as a topic marker (In May.... In August...). This is OK, unless the exercise you are doing specifically asks you to use は to emphasize the contrast. (I also think the use of が is putting emphasis on the people, instead of the months so it is a further complication. It also sounds a bit too simplified... "people in the sea").