We have collected both dwarf and regular seahorses, in Sarasota Bay and the intercoastal. They're usually in 3-4' of water, in weeds (frequently turtle grass, but pretty much any rooted weeds). We just run a long-handled net along the bottom. Sometimes there are quite a few, sometimes none, so don't give up on a place if you don't find them the first time. They're there regardless of water temp, we've caught them in January (water temp probably 65) and August (water temp close to 90).
The full size horses usually eat frozen brine shrimp within a few days.
Do watch, there are toadfish and scorpionfish in the same types of habitat, don't go without shoes, and look at what you're taking out of the net!
Should mention a saltwater fishing license is needed...