I thought i'd share my experience after having been there.
The best place to be would be, according to locals, the Tobago island, where you do have nice reefs and clear water. The Speyside beach came highly recommended (more than some other spots that got famous after Cousteau, but degradated since then).
If you're stuck in Trinidad like me, well there's a part of the coast covered with mangroves that is a bit special (i did snorkel mangrove in Bali and liked it though).
My free time was very limited, but i went to some of the northern beaches:
1/ Maracas Bay.
Quite a large & beautiful beach, with a fishing village on the rocky west side. That's the place which came recommended for snorkelling as you could see baby sharks there.
Wheather was bad so i had terrible viz and not sufficient time to properly investigate, but there's probably some potential.
One thing to note is they serve "bake'n'shark" (shark burgers) on that beach, and you can see the reminders of shark heads on the beach, they attract the local vultures, plus quite a few pelicans.
I was sad to see one baby hammerhead's head in particular... But at least the locals are eating the whole fish, not just taking fins.
2/ Las Cuevas Bay
Beautiful beach, and here the rocky side + fishing village is on the eastern side.
On this one i spent more time snorkelling, but even going ~200/300 meters, you don't get much depth (3 meters max), and the viz was also terrible (due to weather? i don't know).
Underwater i saw small black fishes and sponges, i'll see if i have any decent picutre, doubt it.
3/ Blanchisseuse Bay
Litterally means "laundry lady" in French, i let you guess the sea conditions then - i haven't been there.
That's it for my very limited exploration (4 hours).
Bottom-line: if you're in Trinidad, it's recommended to take the cheap flights to Tobago (20+ flights daily, 25mins distance, 48 USD round-trip).
cheers
Nicolas