There are actually three separate dive sites at Wetherill - Sandy Bottom Cove (the one with the boat ramp on the right hand side), West Cove (the one with a "goat path" down to the beach on the left hand side), and Bull Point (the rocky beach out by the fort itself - past the DEM office).
In general, if you're diving either of the coves (Sandy Bottom or West), stick to the walls - there's not much in the middle but silty sand (occasional tires/concrete blocks may have some life on them and you could see some horseshoe crabs and tube-dwelling anemones). Check the eelgrass for juvie tropicals, seahorses and pipefish.
You may get better vis on an outgoing tide, but it's definitely diveable on either. Both coves are fairly well protected from strong current. Bull Point, OTOH, is a bit more exposed, particularly if you get out farther. I'd try to dive that at slack - or, if you want, dive it as a drift dive on an incoming tide and exit around the point at the marina by the DEM building. If you dive Bull Point, watch your entry and exit - the beach consists of round cobbles and it's easy to slip and fall. Enjoy!