Weather in Martinique in June is glorious, though keep in mind that June is at the beginning of the hurricane season. Another reason why June is also a great time to travel there is because of the fewer crowds. If you were to go to Martinique between November and May, you wouldn't like it because europeans flock there in mass.
The worst of the weather and crowds comes in right after June and I too agree that anytime between May and June is great and affordable. Martinique's weather in June is in the high 80's. By the way, The most beautiful beaches for swimming and snorkelling are in the south in places like St. Luce.
You should know that there really are no “seasons†in Martinique and temperatures throughout the years are pretty consistent...there's about a change only about 5 degrees from summer to winter. Most tourists come to Martinique during the winter (from December to April) which means exorbitant prices.