Baermann extraction probably will miss many spp. I found it best to use both that method, which depends entirely on nematode movement, and simple decanting-sieving with seawter (use stainless steel sieve -- phosphor bronze will be destroyed by seawater). Preserve specimens in seawater with about 5% formaldehyde in it. Good luck-- some of the marine nematodes are among the most bizarre-looking animals on earth.