I'm guessing you can, but the sea water we can easily get tends to be polluted with stuff like mercury and other heavy metals... so.. I guess if it's not polluted, you can use it. But salt mixes for marine set up not that expensive, about RM20++ you get a 2kg pack and I only used half to get the desired salinity(I don't know if it's the correct salinity). Anyone with marine experience can tell me how to test to get the ppt of salinity? Aiming for 34ppt. I've been only using a hydrometer to get gravity measurements.. you know the one that looks like a floating thermometer?

I plan to suck the larvae out with a pippete, or i'll try to get a turkey baster as the pippete's opening may be too small when it grows. If they're large enough i'll use a fine net. And I'll put them in plastic cups to slowly acclimatize them to freshwater again. This method has been described on
caridina.japonica.online.fr which I think is pretty good but labour intensive!