Dug through the bookings for the Islander. Looks like they're doing their schedule based on the Tampa port availability. I believe Tampa can only hold three ships maximum. For the most part, Tampa only has the Carnival Paradise, Carnival Pride(winters only), RCCL Radiance of the Seas & Enchantment of the Seas, NCL Jade(winters only) and Celebrity Constellation(winters only). The majority of the 5-day sailings are Cozumel & Key West.
Schedules through September:
4-day cruises: June 14 & 28, July 12 & 26, August 9 & 23, Sept. 6 & 20
5-day cruises(Key West & Cozumel): June 18 & 23, July 7, 21 & 30, August 4, 13, 18 & 27, Sept. 1, 10, 15, 24 & 29
5-day cruises(Key West & Progesso): July 2
5-day cruises(Progresso & Cozumel): July 16