Seahorses are masters of disguise. They tend to be found near structure, and are able to blend in perfectly.. They stretch and contort their bodies much more than many people realize. For every seahorse spotted by a diver there usually are several more that evade detection.
Seahorses prefer shallower water where there is abundant food. Reefs, especially deeper reefs, are not their preferred habitat. Weedy patches, mangroves, old wooden structures and similar places are where they are most likely to be found. This tends to be true on every island with which I am familiar, and is certainly true here in NJ, where a hour's dive in an appropriate habitat will often reveal a dozen or more seahorses in late summer.
In quiet Caribbean coves I have seen numbers of them in less than two feet of water. They often are extremely numerous around old docks and rotting bulkheads from New England to the Caribbean.
They are among the rewards garnered by those who dive in shallow non-traditional areas.