Larger Stuff? Like you said, you know about the Roatan Shark dive...
On a broader sense, take a breath, do some diving, sharpen your observational skills.
There's lots of "larger stuff" swimming right by you.
We all begin diving with the same skills.
We can't see or find anything. Thus- we gravitate towards easily seen and identifiable critters. Lobsters, Crabs (Hey... I've seen those in a restaurant!), bigger fish like Baracuda, Groupers, and similar.
It is so insidious that
no diver can know what he isn't seeing. I have had students that were so keyed up with
"Wooowooo... I'm diving!" (kind of like the first time you have sex) that they couldn't see a 10" lobster on the sandy bottom from 4 feet away. Not all that uncommon.
Sometimes we are so eager to see something, we'll pay for the priveledge to see Sharks attracted to feed before us. We might go to a resort that features Dolphin encounters. Both of these (and similar are called "Canned Fish" acts or "Rodeos") you will someday realize are pretty dreadful.
Tobago? Only one Dive Op that I know of that could give you a remote chance of seeing Mantas- get 40+ logged small boat dives (wooden ladders) and drift (heavy current) recoveries before you go play with those guys.
Grenada? The only place to see the real deal is the far NE barrier islands- getting there will cost serious money and two plus days of diving with specific dive ops before they will bother to trust you.
Roatan? There is a Canned Shark Dive and a Dolphin Pester. For what they are, two of the best I have ever seen in terms of dive breifing and dive-op infrastructure. You can often see (and snorkel with) free dolphins on the South Side at First Bight (East of CCV). See Pilot Whales from New Years at
http://www.docksidedivecenter.com/WeeklyLog122906.html on just a regular dive boat trip.
Roatan and Belize do offer some potential for Whale Sharks, but specific times of year are an issue, phases of the moon, plus the dive-op itself has to be doing some serious networking. The most Whale Sharks ever dove in the Bay Islands was in the day that CCV had a float plane diong the spotting. Go for the diving, if you get a Whale Shark- consider that you've won the lottery.
No arranged dive will come close to your first incidental encounter in the wild. You may have seen 100 sharks being fed on 8 different
Canned Shark dives, but
that one dive when you are at 60 feet off of San Sal and a 12' Bull Shark swims along 100 feet away and just ignores you...
that is a memory.
Go to some place with great stuff to see. Concentrate on that life that is 6" down to microscopic. Practice your buoyancy, your observational skills and diving. You'll see the "big stuff" soon enough!
If you go looking for "love", you'll likely not find it!