I saw it in the sea very close to the beach in front of Starfish Hotel in St Lucia. It had some spines along its back and it seemingly had fins on each side that split open into 4. It used its front two fins to dig through the sand. It had white spots, and the main colour of it was very dark brown - black. The fins on the fish were very thin and flimsy-looking. From tail to head was probably around a foot long, and the end of one fin to another was around 15-20 cm. Here is a link to a video I took seeing as the picture may not be clear enough- https://drive.google.com/file/d/1PzjgzAaK6B51jg9HHNO4SQyorepsdKLB/view?usp=sharing
Based on the location, size, behavior, and shape (particularly of the pectoral fins) this appears to be a flying (or helmet) gurnard (Dactylopterus volitans) with its "wings" folded.
These range of these fish includes the Caribbean and they are known to use the forepart of their pectoral fins (i.e. the free extra 'fin' near their head) for "exploring the bottom" 1-3, which fits the behavior you described.
The size is also reasonable since they are said to reach up to 50 cm in length.
Image © slebris from iNaturalist some rights reserved
Roux C; Dactylopteridae. In: Miller, P. J., Whitehead, P. J. P., Bauchot, M. L., Hureau, J. C., Nielsen, J., & Tortonese, E. (1986). Fishes of the North-eastern Atlantic and the Mediterranean. Vol. III. Richard Clay Ltd, Bungay, United Kingdom.