How about celocanths, which are actually the anscestors of dinosaurs, and still around much as they were back then.
Edit: althought they are not saltwater anymore lung fish like the african lungfish were around before lobefinned fish like the celocanth and even before cartilagenous fish as well if I recall correctly, then there are salps which are not fish but are fish like and the earliest segmented cordates, they can be found in shallow waters in florida althought i think you'll have a hard time getting your hands on them.