Hi!
You might want to reconsider the Carib for a Summer trip: that is the middle of hurricane season and, with the exception of the very southern part of the Carib, your trip runs the risk of cancellation, bad weather either before or after a hurricane, or, much worse, being on board when a hurricane passes through. Of course, the captain will have the boat in port, if that happens. But remember the "Dancer" and Belize!
The Aggressors are a class fleet and you will probably love the accommodations, crew, and ammenities--not to mention the diving! Fiji is a delight and I can recommend that trip. However, remember that our Northern Hemisphere Summer is their Southern Hemisphere Winter. Fortunately, the "dry" season on Fiji is May-October and, since it is Winter there, it is the cool season. The bad side: water temps can be as low as 74 deg. F (but they can be in the high 70's also) and the surface can be a little choppy. The good side: vis. is fantastic--no algae, etc. Surface temps will be typical tropical 80's+.
The sealife around Fiji is abundant and gorgeous! You will, no doubt, have many opportunities to spot what I consider to be some of the most beautiful fish in the ocean seas: the Clown Triggerfish. If you are really lucky, you will run across a Ribbon Eel. They are one of the most spectacularly colored fish you will ever see. And there is much, much more.
Never been to Tahiti (great loss that needs to be addressed), so I cannot make a recommendation. However, if you go on an Aggressor, it will likely be a fantastic trip!
Have fun!
Scorpionfish