I use sea water too, free and no mucking around with salt.
Get acoke bottle and cut the base off. Drill a hole in the cap and push a tube through. Seal the hole. Connect an airhose to the tube. Fill it with sea water and put in 1-2 teaspoon of brinshrimp egg. Turn air pump on and wait for 30 hrs for eggs to hatch.
Turn air off and let bbs settle to the bottom.
Disconnect airhose from pump to drain bbs out through a brine shrimp seive.
Make up another bottle as above to put the bbs in and use for feeding.
Wash bottle and hatch another lot with new sea water .
Hatching photo
Storing hatched BBS
Hope that help,
ron